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ffield file — ReaxFF force field definition

The ffield input file defines all force field parameters used by ReaxFF.

The first line of the ffield file contains a force field identifier (see Example 2.8 in the). The remainder of the file is divided into seven sections, corresponding to different interaction types.

Structure of the ffield file

Seven sections in order:

  1. General parameters
  2. Atom parameters
  3. Bond parameters
  4. Off-diagonal terms
  5. Valence angle parameters
  6. Torsion angle parameters
  7. Hydrogen bond parameters

Each section begins with a count of the parameter entries it contains, followed by parameter identifiers and numeric values.

Figure 1 illustrates the overall layout of the ffield file and its sections.

1. General parameters

The general parameter section contains parameters that affect all interactions, regardless of atom type.

Format

  • First line:
  • npar — number of general parameters (i3 format)
  • Followed by npar lines, each containing:
  • parameter value
  • parameter identifier

Important parameters

Two parameters in this section strongly affect performance:

  • Upper Taper radius
    Controls the non-bonded cutoff distance.
  • Bond order cutoff
    Sets the bond order threshold above which atoms are considered bonded.

⚠️ Changing these parameters can significantly speed up ReaxFF but requires re-parameterization, as they strongly affect force-field behavior.

2. Atom parameters

The atom parameter section defines element-specific properties.

Format

  1. Number of atom types
  2. Four lines of parameter identifiers
  3. For each atom type:
  4. One line with atom name + 8 parameters (1x,a2,8f9.4)
  5. Three continuation lines with 8 parameters each (3x,8f9.4)

Notes

  • Negative values for bond radii (cov.r, cov.r2, cov.r3) disable the corresponding bond-order contribution.
  • Example: if only cov.r is positive, only σ-bonds are considered for that atom.

Table 1 describes the carbon (C) atom parameters from Example 2.8.

Table 1: Carbon atom parameters (C)

Value Identifier Description
1.3826 cov.r σ-bond covalent radius
4.0000 valency Valency
12.0000 a.m. Atomic mass
2.0195 Rvdw van der Waals radius
0.0763 Evdw van der Waals dissociation energy
0.8712 gammaEEM EEM shielding
1.2360 cov.r2 π-bond covalent radius
4.0000 #el. Number of valence electrons
10.6359 alfa van der Waals parameter
1.9232 gammavdW van der Waals shielding
4.0000 valency Valency for 1,3-BO correction
40.5154 Eunder Undercoordination energy
5.7524 chiEEM EEM electronegativity
6.9235 etaEEM EEM hardness
1.1663 cov.r3 Double π-bond covalent radius
0.0000 Elp Lone-pair energy
200.049 Heat inc. Heat of formation increment
6.1551 13BO1 Bond order correction
28.6991 13BO2 Bond order correction
12.1086 13BO3 Bond order correction
-14.1953 ov/un Over-/undercoordination
3.5288 vval1 Valence angle energy
6.2998 vval2 Valence angle energy
2.9663 vval3 Valence angle energy

n.u. identifiers are omitted.

3. Bond parameters

This section defines bonded interactions between atom pairs.

Format

  1. Number of bond types
  2. Two lines of parameter identifiers
  3. For each bond type:
  4. Line 1: atom-type pair + 8 parameters (2i3,8f9.4)
  5. Line 2: 8 continuation parameters (6x,8f9.4)

ReaxFF will terminate immediately if a bond type appears during a simulation that is not defined in the ffield file.

Table 2 summarizes the carbon–carbon (C–C) bond parameters from Example 2.8.

Table 2: C–C bond parameters

Value Identifier Description
152.0140 Edis1 σ-bond dissociation energy
104.0507 Edis2 π-bond dissociation energy
72.1693 Edis3 Double π-bond dissociation energy
0.2447 pbe1 Bond energy
-0.7132 pbo5 Double π bond order
1.0000 13corr 1,3 bond-order correction
23.5135 pbo6 Double π bond order
0.3545 kov Overcoordination penalty
0.1152 pbe2 Bond energy
-0.2069 pbo3 π bond order
9.2317 pbo4 π bond order
-0.1042 pbo1 σ bond order
5.9159 pbo2 σ bond order
1.0000 ovcorr Overcoordination BO correction

4. Off-diagonal terms

Off-diagonal parameters override default combination rules for:

  • Bond order interactions
  • van der Waals interactions

Format

  • Number of off-diagonal types
  • Parameter identifiers (same line)
  • One line per type (2i3,6f9.4)

Example: 1 2 defines C–H off-diagonal parameters.

Table 3 lists the C–H off-diagonal parameters from Example 2.8.

Table 3: C–H off-diagonal parameters

Value Identifier Description
0.0404 Ediss vdW dissociation energy
1.8583 Rvdw vdW radius
10.3804 alfa vdW parameter
1.0376 cov.r σ-bond covalent radius
-1.0 cov.r2 π-bond covalent radius
-1.0 cov.r3 Double π-bond covalent radius

Negative values disable π and double-π bond orders.

5. Valence angle parameters

Defines angle interactions between bonded atom triplets.

Format

  • Number of valence angles
  • Parameter identifiers
  • One line per angle (3i3,7f9.4)

Angles not defined in the ffield file are ignored.

Table 4 describes the C–C–C valence angle parameters.

Table 4: C–C–C valence angle parameters

Value Identifier Description
70.2140 Thetao 180° − equilibrium angle
14.0458 ka First force constant
2.0508 kb Second force constant
0.0000 pconj Valence conjugation
0.0000 pv2 Undercoordination
35.9933 kpenal Penalty energy
1.0400 pv3 Energy/bond order

This corresponds to an equilibrium angle of 109.786° for C–C–C σ bonds.

6. Torsion angle parameters

Defines dihedral interactions.

Identification modes

  1. Central-bond based (e.g. 0 1 1 0 → all C–C torsions)
  2. Four-atom specific (e.g. 1 1 1 2 → C–C–C–H)

Four-atom identifiers override central-bond definitions.

Table 5 lists C–C–C–C torsion parameters from Example 2.8.

Table 5: C–C–C–C torsion parameters

Value Identifier Description
0.0000 V1 V1 torsion barrier
28.8256 V2 V2 torsion barrier
0.1796 V3 V3 torsion barrier
-4.6957 V2(BO) V2 bond-order dependence
-1.3130 vconj Torsion conjugation

Torsions not defined in the ffield file are ignored.

7. Hydrogen bond parameters

Defines hydrogen bond interactions.

Format

  • Number of hydrogen bond types
  • Parameter identifiers
  • One line per hydrogen bond (3i3,4f9.4)

Example identifier 1 2 1 corresponds to a C–H···C hydrogen bond.

Table 6 summarizes C–H···C hydrogen bond parameters.

Table 6: C–H···C hydrogen bond parameters

Value Identifier Description
2.0347 Rhb Equilibrium H-bond distance
0.0000 Dehb Dissociation energy
4.9076 vhb1 H-bond / bond-order term
4.2357 vhb2 H-bond parameter

Hydrogen bonds not defined in the ffield file are ignored.