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


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


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;#
       cov r3;Elp;Heat inc.;n.u.;n.u.;n.u.;n.u.
 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.


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.


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


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


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


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


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


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