Force field format specification¶

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.

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.

Bonds

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.

Angles

Torsions

Hydrogen bonds