Force field file format

See sample prm-file as well as topology xyz-file and example of LAMMPS MD input/output as zip-archive.

The structure of prm-file is as follows (see LAMMPS manual for keywords):

# comment line

any LAMMPS input

special_bonds lj 0 0 1 coul 0 0 1 (1 1 for Dreiding, 1/2 5/6 for AMBER, 0 0 for CHARMM)

    type  mass    class symbol coordination  charge    description
mass  1   1.008   #  1    H         1        0.1475    HC-alpha/beta THIOPHENE
mass  2  31.972   #  2    S         2        0.0090    S polyTHIO centr-HR
...

pair_style lj/cut/coul/cut 10 10 (refine parameters and consider other coul styles)

            i ≤ j  epsilon   sigma (here ij are types)
pair_coeff  1   1   0.0300   2.4200
pair_coeff  2   2   0.2500   3.5500
...

bond_style harmonic

             id      K    bondlength   i ≤ j (from now on ijkl are classes)
bond_coeff    1    367.0    1.082   #  1   3
bond_coeff    2    367.0    1.082   #  1   4
...

angle_style harmonic
(it is angle between ji and jk, so that j is central atom)
              id    K     angle      i   j   k (i≤k)
angle_coeff    1   60.0   91.00   #  3   2   3
angle_coeff    2   60.0  119.00   #  1   3   2
...

dihedral_style opls
(dihedral is angle between ijk and jkl planes, so that jk is central bond)
                 id     K1     K2      K3      K4        i   j ≤ k   l
dihedral_coeff    1   0.000   7.250   0.000   0.000   #  3   2   3   1
dihedral_coeff    2   0.000   7.250   0.000   0.000   #  3   2   3   3
...

improper_style harmonic
(improper is angle between ijk and jkl planes, i is central atom bonded to j,k,l)
                 id     K   angle     i   j ≤ k   l
improper_coeff    1     2.2  180   #  3   2   4   1
improper_coeff    2     2.2  180   #  3   2   4   3
...