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