Force field format specification
********************************

Each force field file consist of following sections:

 
| Section name                                                                                         | N of params                                                                                          | N of header lines                                                                                    | N of block keys                                                                                      | block keys                                                                                           |
 
| General                                                                                              | 41                                                                                                   | 1                                                                                                    |                                                                                                      |                                                                                                      |
 
| Atoms                                                                                                | 32                                                                                                   | 4                                                                                                    | 1                                                                                                    | atom type name                                                                                       |
 
| Bonds                                                                                                | 16                                                                                                   | 2                                                                                                    | 2                                                                                                    | atom type index                                                                                      |
 
| Off-diagonal                                                                                         | 6                                                                                                    | 1                                                                                                    | 2                                                                                                    | atom type index                                                                                      |
 
| Angles                                                                                               | 7                                                                                                    | 1                                                                                                    | 3                                                                                                    | atom type index                                                                                      |
 
| Torsions                                                                                             | 7                                                                                                    | 1                                                                                                    | 4                                                                                                    | atom type index                                                                                      |
 
| Hydrogen bonds                                                                                       | 4                                                                                                    | 1                                                                                                    | 2                                                                                                    | atom type index                                                                                      |
 


Format
======

The force field file begins with a description line that, in turn, may
optionally begin with a list of keywords between square brackets, for
example (for an ACKS2+eReaxff force-field):

   [ ereaxff acks2 ] Reactive MD-force field for Ethylene Carbonate and Li

Each section starts with one or more header line containing, on the
first line, the number of blocks in the section, possibly followed by
description of the parameters. The number of header lines is supposed
to match the number lines in a block of the corresponding section (4
in atoms, 2 in bonds and 1 in every other section). The additional
header lines after the first are skipped when reading the force field
file.

**General parameters**

The header of this section starts with *npar*, the number of general
parameters present in the force field file. The ehader is followed by
*npar* lines each containing a parameter value followed by a comment,
for example:

    39     ! Number of general parameters
   50.0000 !Overcoordination parameter

**Atoms**

The atomic parameters section starts with the number of atom types
present in the force field, followed by three additional header lines
and the blocks of parameters, one block per atom type. Each block
consists of 4 lines starting with a line containing the atom name and
8 parameter values with the (1x,a2,8f9.4) format followed by three
lines with 8 parameter values each, with the (3x,8f9.4) format, for
example:

   3    ! Nr of atoms; cov.r; valency;a.m;Rvdw;Evdw;gammaEEM;cov.r2;#
          alfa;gammavdW;valency;Eunder;Eover;chiEEM;etaEEM;n.u.
          cov r3;Elp;Heat inc.;n.u.;n.u.;n.u.;n.u.
          ov/un;val1;n.u.;val3,vval4
    C    1.3817   4.0000  12.0000   1.8903   0.1838   0.   9000   1.1341   4.0000
         9.7559   2.1346   4.0000  34.9350  79.5548   5.   9666   7.0000   0.0000
         1.2114   0.0000 202.5551   8.9539  34.9289  13.   5366   0.8563   0.0000
        -2.8983   2.5000   1.0564   4.0000   2.9663   0.   0000   0.0000   0.0000
    H    0.7853   1.0000   1.0080   1.5904   0.0419   1.0206  -0.1000   1.0000
         9.3557   5.0518   1.0000   0.0000 121.1250   5.3200   7.4366   1.0000
        -0.1000   0.0000  62.4879   1.9771   3.3517   0.7571   1.0698   0.0000
       -15.7683   2.1488   1.0338   1.0000   2.8793   0.0000   0.0000   0.0000
   ... one more block ...

**Bonds, angles, etc.**

In the remaining sections, a block key consists of two or more integer
numbers, each of them referring to the atomic block with this index.
The number of integers in the key depends on the block type (two for
bonds, three for valence angles, etc.). For instance, the bond
parameters block below corresponds to the C-H bond for the atoms block
shown above.

The bond parameters section starts with the number of bond types
followed by one additional comment line. The first line of the block
has the (2i3,8f9.4) format and the second (6x,8f9.4):

     6      ! Nr of bonds; Edis1;  LPpen;n.u.;pbe1;pbo5;13corr;pbo6
             pbe2;pbo3;pbo4;n.u.;pbo1;pbo2;ovc  orr
     1  1 156.5953 100.0397  80.0000  -0.8157  -0.4591   1.0000  37.7369   0.4235
            0.4527  -0.1000   9.2605   1.0000  -0.0750   6.8316   1.0000   0.0000
   ... five more blocks ...

For the rest of the sections, the format remains similar to the bonds
section, except that they do not have additional header lines and the
Fortran format may be slightly different: (2i3,6f9.4) for off-
diagonal,(3i3,7f9.4) for valence angles, (4i3,7f9.4) for torsion
angles, and (3i3,4f9.4) for hydrogen bonds.


Equation Reference
==================

In the tables below the ReaxFF parameters are listed with their
corresponding equation numbers from the "SCM developer notes" which
have mostly technical relevance. For a good introduction to the
meaning of ReaxFF parameters we advise the initial ReaxFF publication
ReaxFF: A Reactive Force Field for Hydrocarbons, A.C.T. van Duin, S.
Dasgupta, F. Lorant, W.A. Goddard, J. Phys. Chem. A. 2001 105 41
9396-9409.

Note:

  The "SCM developer notes" contain very detailed information about
  all the ReaxFF equations, including some information that concerns
  SCM's implementation of ReaxFF. That SCM-specific information can be
  ignored.

**General**

Of particular interest are the upper taper radius parameter (#13),
which describes the non-bonded cutoff radius, and the bond order
cutoff (#30), which describes the bond order threshold, above which
atoms are considered connected. Both these parameters may have a major
impact on the ReaxFF calculation speed; decreasing the taper radius or
increasing the bond order cutoff can make ReaxFF run considerably
faster. These parameters, however, have a significant impact on the
force description and should not be changed without re-
parameterization of other parts of the force field.

 
| Index                                                                                                | Name in Eq                                                                                           | Equation                                                                                             | Comment                                                                                                                                                                                                  |
 
| 1                                                                                                    | p_boc1                                                                                               | 4c                                                                                                   | Overcoordination parameter                                                                                                                                                                               |
 
| 2                                                                                                    | p_boc2                                                                                               | 4d                                                                                                   | Overcoordination parameter                                                                                                                                                                               |
 
| 3                                                                                                    | -p_coa2                                                                                              | 15                                                                                                   | Valency angle conjugation parameter                                                                                                                                                                      |
 
| 4                                                                                                    | p_trip4                                                                                              | 20                                                                                                   | Triple bond stabilization parameter                                                                                                                                                                      |
 
| 5                                                                                                    | p_trip3                                                                                              | 20                                                                                                   | Triple bond stabilization parameter                                                                                                                                                                      |
 
| 6                                                                                                    | k_c2                                                                                                 | 19                                                                                                   | C2-correction                                                                                                                                                                                            |
 
| 7                                                                                                    | p_ovun6                                                                                              | 12                                                                                                   | Undercoordination parameter                                                                                                                                                                              |
 
| 8                                                                                                    | p_trip2                                                                                              | 20                                                                                                   | Triple bond stabilization parameter                                                                                                                                                                      |
 
| 9                                                                                                    | p_ovun7                                                                                              | 12                                                                                                   | Undercoordination parameter                                                                                                                                                                              |
 
| 10                                                                                                   | p_ovun8                                                                                              | 12                                                                                                   | Undercoordination parameter                                                                                                                                                                              |
 
| 11                                                                                                   | p_trip1                                                                                              | 20                                                                                                   | Triple bond stabilization energy                                                                                                                                                                         |
 
| 12                                                                                                   | n/a                                                                                                  | 21                                                                                                   | Lower Taper-radius                                                                                                                                                                                       |
 
| 13                                                                                                   | R_cut                                                                                                | 21                                                                                                   | Upper Taper-radius                                                                                                                                                                                       |
 
| 14                                                                                                   | p_fe1                                                                                                | 6a                                                                                                   | Fe dimer correction                                                                                                                                                                                      |
 
| 15                                                                                                   | p_val6                                                                                               | 13c                                                                                                  | Valency undercoordination                                                                                                                                                                                |
 
| 16                                                                                                   | p_lp1                                                                                                | 8                                                                                                    | Valency angle/lone pair parameter                                                                                                                                                                        |
 
| 17                                                                                                   | p_val9                                                                                               | 13f                                                                                                  | Valency angle parameter                                                                                                                                                                                  |
 
| 18                                                                                                   | p_val10                                                                                              | 13g                                                                                                  | Valency angle parameter                                                                                                                                                                                  |
 
| 19                                                                                                   | p_fe2                                                                                                | 6a                                                                                                   | Fe dimer correction                                                                                                                                                                                      |
 
| 20                                                                                                   | p_pen2                                                                                               | 14a                                                                                                  | Double bond/angle parameter                                                                                                                                                                              |
 
| 21                                                                                                   | p_pen3                                                                                               | 14b                                                                                                  | Double bond/angle parameter: overcoord                                                                                                                                                                   |
 
| 22                                                                                                   | p_pen4                                                                                               | 14b                                                                                                  | Double bond/angle parameter: overcoord                                                                                                                                                                   |
 
| 23                                                                                                   | p_fe3                                                                                                | 6a                                                                                                   | Fe dimer correction                                                                                                                                                                                      |
 
| 24                                                                                                   | p_tor2                                                                                               | 16b                                                                                                  | Torsion/BO parameter                                                                                                                                                                                     |
 
| 25                                                                                                   | p_tor3                                                                                               | 16c                                                                                                  | Torsion overcoordination                                                                                                                                                                                 |
 
| 26                                                                                                   | p_tor4                                                                                               | 16c                                                                                                  | Torsion overcoordination                                                                                                                                                                                 |
 
| 27                                                                                                   | p_elho                                                                                               | 26a                                                                                                  | eReaxFF                                                                                                                                                                                                  |
 
| 28                                                                                                   | p_cot2                                                                                               | 17b                                                                                                  | Conjugation                                                                                                                                                                                              |
 
| 29                                                                                                   | p_vdW1                                                                                               | 23b                                                                                                  | VdW shielding                                                                                                                                                                                            |
 
| 30                                                                                                   | cutoff * 100                                                                                         | 3a,b                                                                                                 | Cutoff for bond order (* 100)                                                                                                                                                                            |
 
| 31                                                                                                   | p_coa4                                                                                               | 15                                                                                                   | Valency angle conjugation parameter                                                                                                                                                                      |
 
| 32                                                                                                   | p_ovun4                                                                                              | 11b                                                                                                  | Overcoordination parameter                                                                                                                                                                               |
 
| 33                                                                                                   | p_ovun3                                                                                              | 11b                                                                                                  | Overcoordination parameter                                                                                                                                                                               |
 
| 34                                                                                                   | p_val8                                                                                               | 13d                                                                                                  | Valency/lone pair parameter                                                                                                                                                                              |
 
| 35                                                                                                   | X_soft                                                                                               | 25                                                                                                   | ACKS2 softness parameter                                                                                                                                                                                 |
 
| 36                                                                                                   | unused                                                                                               | n/a                                                                                                  | n/a                                                                                                                                                                                                      |
 
| 37                                                                                                   | p_val                                                                                                | 27 via n_el                                                                                          | eReaxFF                                                                                                                                                                                                  |
 
| 38                                                                                                   | n/a                                                                                                  | 13d                                                                                                  | if 1: remove delta_j term for non-C-C-C angles and where none of the atoms is N                                                                                                                          |
 
| 39                                                                                                   | p_coa3                                                                                               | 15                                                                                                   | Valency angle conjugation parameter                                                                                                                                                                      |
 
| 40                                                                                                   | n/a                                                                                                  | 20                                                                                                   | Condition to turn triple bond option: vpar(40) == 1                                                                                                                                                      |
 
| 41                                                                                                   | n/a                                                                                                  | 26 via Tap(R)                                                                                        | eReax-specific taper radius for interactions with/between electrons and holes                                                                                                                            |
 

**Atoms**

If negative values are provided to either of the three bond radii
(**sigma, pi, and double pi**) the bond order contributions are
ignored for that atom.

 
| Index                                                                                                | Name in Eq                                                                                           | Equation                                                                                             | Comment                                                                                                                                                                                                  |
 
| 1                                                                                                    | r_0^sigma                                                                                            | 2                                                                                                    | **Sigma bond covalent radius**                                                                                                                                                                           |
 
| 2                                                                                                    | Val_i                                                                                                | 3a, 4b, 5, 9a                                                                                        | Valency                                                                                                                                                                                                  |
 
| 3                                                                                                    | n/a                                                                                                  | 9a                                                                                                   | Atomic mass                                                                                                                                                                                              |
 
| 4                                                                                                    | r_vdW                                                                                                | 23a                                                                                                  | van der Waals radius                                                                                                                                                                                     |
 
| 5                                                                                                    | D_ij                                                                                                 | 23a                                                                                                  | van der Waals dissociation energy                                                                                                                                                                        |
 
| 6                                                                                                    | gamma_i                                                                                              | 24                                                                                                   | gammaEEM; EEM shielding                                                                                                                                                                                  |
 
| 7                                                                                                    | r_0^pi                                                                                               | 2                                                                                                    | **Pi bond covalent radius**                                                                                                                                                                              |
 
| 8                                                                                                    | Val_i^e                                                                                              | 7, 8, 9                                                                                              | Number of valence electrons                                                                                                                                                                              |
 
| 9                                                                                                    | alpha_ij                                                                                             | 23b                                                                                                  | van der Waals parameter                                                                                                                                                                                  |
 
| 10                                                                                                   | 1/gamma_w                                                                                            | 23b                                                                                                  | van der Waals shielding                                                                                                                                                                                  |
 
| 11                                                                                                   | Val_j^angle                                                                                          | 16c, 13c                                                                                             | Valency for 1,3-BO correction                                                                                                                                                                            |
 
| 12                                                                                                   | p_ovun5                                                                                              | 12                                                                                                   | Undercoordination energy                                                                                                                                                                                 |
 
| 13                                                                                                   | p_i^xel2                                                                                             | 26                                                                                                   | eReaxFF, atom type parameter                                                                                                                                                                             |
 
| 14                                                                                                   | chi_i                                                                                                | 24, 25                                                                                               | EEM electronegativity                                                                                                                                                                                    |
 
| 15                                                                                                   | eta_i                                                                                                | 24, 25                                                                                               | EEM hardness                                                                                                                                                                                             |
 
| 16                                                                                                   | n/a                                                                                                  | n/a                                                                                                  | Donor or acceptor switch in H-bonds                                                                                                                                                                      |
 
| 17                                                                                                   | r_0^pipi                                                                                             | 2                                                                                                    | **Double pi bond covalent radius**                                                                                                                                                                       |
 
| 18                                                                                                   | p_lp2                                                                                                | 10                                                                                                   | Lone pair energy                                                                                                                                                                                         |
 
| 19                                                                                                   | n/a                                                                                                  | n/a                                                                                                  | Atomic heat of formation                                                                                                                                                                                 |
 
| 20                                                                                                   | p_boc4                                                                                               | 4e,f                                                                                                 | Bond order correction                                                                                                                                                                                    |
 
| 21                                                                                                   | p_boc3                                                                                               | 4e,f                                                                                                 | Bond order correction                                                                                                                                                                                    |
 
| 22                                                                                                   | p_boc5                                                                                               | 4e,f                                                                                                 | Bond order correction                                                                                                                                                                                    |
 
| 23                                                                                                   | C_i                                                                                                  | 25                                                                                                   | Atomic softness cutoff parameter                                                                                                                                                                         |
 
| 24                                                                                                   | alpha, alpha_i                                                                                       | 26, 26a                                                                                              | eReaxFF, constant, dependent on atom type                                                                                                                                                                |
 
| 25                                                                                                   | p_ovun2                                                                                              | 12                                                                                                   | Valence angle parameter                                                                                                                                                                                  |
 
| 26                                                                                                   | p_val3                                                                                               | 13b -> 13a                                                                                           | Valence angle parameter                                                                                                                                                                                  |
 
| 27                                                                                                   | beta, beta_i                                                                                         | 26a                                                                                                  | eReaxFF, constant, dependent on atom type                                                                                                                                                                |
 
| 28                                                                                                   | Val_i^'boc                                                                                           | 3b                                                                                                   | Number of lone pairs                                                                                                                                                                                     |
 
| 29                                                                                                   | p_val5                                                                                               | 13b                                                                                                  | Valence angle parameter                                                                                                                                                                                  |
 
| 30                                                                                                   | p_c1                                                                                                 | 23c                                                                                                  | Inner wall vdW repulsion parameter                                                                                                                                                                       |
 
| 31                                                                                                   | p_c2                                                                                                 | 23c                                                                                                  | Inner wall vdW repulsion parameter                                                                                                                                                                       |
 
| 32                                                                                                   | p_c3                                                                                                 | 23c                                                                                                  | Inner wall vdW repulsion parameter                                                                                                                                                                       |
 
| 33                                                                                                   | C_i                                                                                                  | 23d                                                                                                  | Lg dispersion parameter                                                                                                                                                                                  |
 
| 34                                                                                                   | R_eij                                                                                                | 23d                                                                                                  | VdW Radius for Lg dispersion correction                                                                                                                                                                  |
 

**Bonds**

 
| 1                                                                                                    | D_e^sigma                                                                                            | 6, 11a                                                                                               | Sigma-bond dissociation energy                                                                                                                                                                           |
 
| 2                                                                                                    | D_e^pi                                                                                               | 6                                                                                                    | Pi-bond dissociation energy                                                                                                                                                                              |
 
| 3                                                                                                    | D_e^pipi                                                                                             | 6                                                                                                    | Double pi-bond dissociation energy                                                                                                                                                                       |
 
| 4                                                                                                    | p_be1                                                                                                | 6                                                                                                    | Bond energy parameter                                                                                                                                                                                    |
 
| 5                                                                                                    | p_bo5                                                                                                | 2                                                                                                    | Double pi bond parameter                                                                                                                                                                                 |
 
| 6                                                                                                    | Val'_i^boc                                                                                           | 3b                                                                                                   | 1,3-Bond order correction                                                                                                                                                                                |
 
| 7                                                                                                    | p_bo6                                                                                                | 2                                                                                                    | Double pi bond order                                                                                                                                                                                     |
 
| 8                                                                                                    | p_ovun1                                                                                              | 11a                                                                                                  | Overcoordination penalty                                                                                                                                                                                 |
 
| 9                                                                                                    | p_be2                                                                                                | 6                                                                                                    | Bond energy parameter                                                                                                                                                                                    |
 
| 10                                                                                                   | p_bo3                                                                                                | 2                                                                                                    | Pi bond order parameter                                                                                                                                                                                  |
 
| 11                                                                                                   | p_bo4                                                                                                | 2                                                                                                    | Pi bond order parameter                                                                                                                                                                                  |
 
| 12                                                                                                   | unused                                                                                               | n/a                                                                                                  | n/a                                                                                                                                                                                                      |
 
| 13                                                                                                   | p_bo1                                                                                                | 2                                                                                                    | Sigma bond order                                                                                                                                                                                         |
 
| 14                                                                                                   | p_bo2                                                                                                | 2                                                                                                    | Sigma bond order                                                                                                                                                                                         |
 
| 15                                                                                                   | delta'_i                                                                                             | 3a                                                                                                   | Uncorrected BO overcoordination                                                                                                                                                                          |
 
| 16                                                                                                   | p_ij^xel1                                                                                            | 27                                                                                                   | e ReaxFF param; for adjusting number of electrons available to host atom                                                                                                                                 |
 

**Off-diagonal**

This section allows for the definition of off-diagonal values for both
bond order and van der Waals pair interactions. By default, ReaxFF
calculates these terms from the combination rules and the atom
parameters (i.e. the default C-H van der Waals radius is
(RvdW[C]*RvdW[H])0.5), but the off-diagonal section allows for the
definition of different values. Any value given in the off-diagonal
section overrules that obtained from the combination rules.

When setting the off-diagonal van der Waals energy to a negative
number, the mixing rule is used to derive this parameter from the
atomic values.

 
| 1                                                                                                    | D_ij                                                                                                 | 23a                                                                                                  | VdW energy                                                                                                                                                                                               |
 
| 2                                                                                                    | r_vdW                                                                                                | 23a                                                                                                  | VdW radius                                                                                                                                                                                               |
 
| 3                                                                                                    | alpha_ij                                                                                             | 23a                                                                                                  | VdW parameter                                                                                                                                                                                            |
 
| 4                                                                                                    | r_0^sigma                                                                                            | 2                                                                                                    | Sigma bond length                                                                                                                                                                                        |
 
| 5                                                                                                    | r_0^pi                                                                                               | 2                                                                                                    | Pi bond length                                                                                                                                                                                           |
 
| 6                                                                                                    | r_0^pipi                                                                                             | 2                                                                                                    | PiPi bond length                                                                                                                                                                                         |
 
| 7                                                                                                    | C_i, C_lg,ij                                                                                         | 23d                                                                                                  | Lg dispersion parameter                                                                                                                                                                                  |
 

**Angles**

 
| 1                                                                                                    | Theta_0,0                                                                                            | 13g                                                                                                  | 180o-(equilibrium angle)                                                                                                                                                                                 |
 
| 2                                                                                                    | p_val1                                                                                               | 13a                                                                                                  | Valence angle parameter                                                                                                                                                                                  |
 
| 3                                                                                                    | p_val2                                                                                               | 13a                                                                                                  | Valence angle parameter                                                                                                                                                                                  |
 
| 4                                                                                                    | p_coa1                                                                                               | 15                                                                                                   | Valence conjugation                                                                                                                                                                                      |
 
| 5                                                                                                    | p_val7                                                                                               | 13c                                                                                                  | Undercoordination                                                                                                                                                                                        |
 
| 6                                                                                                    | p_pen1                                                                                               | 14b -> 14a                                                                                           | Penalty energy                                                                                                                                                                                           |
 
| 7                                                                                                    | p_val4                                                                                               | 13b                                                                                                  | Valence angle parameter                                                                                                                                                                                  |
 

**Torsions**

 
| 1                                                                                                    | V_1                                                                                                  | 16a                                                                                                  | V1-torsion barrier                                                                                                                                                                                       |
 
| 2                                                                                                    | V_2                                                                                                  | 16a                                                                                                  | V2-torsion barrier                                                                                                                                                                                       |
 
| 3                                                                                                    | V_3                                                                                                  | 16a                                                                                                  | V3-torsion barrier                                                                                                                                                                                       |
 
| 4                                                                                                    | p_tor1                                                                                               | 16a                                                                                                  | Torsion angle parameter                                                                                                                                                                                  |
 
| 5                                                                                                    | p_cot1                                                                                               | 17a                                                                                                  | Conjugation energy                                                                                                                                                                                       |
 
| 6                                                                                                    | unused                                                                                               | n/a                                                                                                  | n/a                                                                                                                                                                                                      |
 
| 7                                                                                                    | unused                                                                                               | n/a                                                                                                  | n/a                                                                                                                                                                                                      |
 

**Hydrogen bonds**

 
| 1                                                                                                    | r_hb^0                                                                                               | 18                                                                                                   | Hydrogen bond equilibrium distance                                                                                                                                                                       |
 
| 2                                                                                                    | p_hb1                                                                                                | 18                                                                                                   | Hydrogen bond energy                                                                                                                                                                                     |
 
| 3                                                                                                    | -p_hb2                                                                                               | 18                                                                                                   | Hydrogen bond/bond order                                                                                                                                                                                 |
 
| 4                                                                                                    | -p_hb3                                                                                               | 18                                                                                                   | Hydrogen bond parameter                                                                                                                                                                                  |
 
