The Force Field File

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.

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^pi	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.

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