I’m planning to use the Hill-Sauer Force Field (doi: 10.1021/j100023a036) to simulate zeolite. This force field is based on the class Ⅱ CFF, so I should use the class2 styles in LAMMPS (version: LAMMPS 64-bit 2Aug2023-MSMPI with Python).
However, for the L-J interactions, the paper used the equation as fig.1, and they set Bij to zero as fig.2. The equation in LAMMPS for lj/class2/coul/long is Fig.3, and I can’t just simply set σ to zero.
So I’m confused about how to set the simulation parameters for the L-J interactions?
The pair_style lennard/mdf takes the LJ parameters in the A/B form, but with the usual 12//6 exponents. Alternatively, you could write the functional form of the potential energy (and its derivative) as an analytical function in your favourite program (e.g. gnuplot), write the tabulated energy and force to a file, and use the pair_style table command to read it. Please refer to the documentation to learn the syntax of the tabulated file.
The closest you can come to having set B_ij to zero is to determine the minimum of the LJ-9-6 potential and set the cutoff to that distance, so that you have a repulsive-only potential.
Tabulation can be difficult to achieve accuracy with.
Another alternative would be to use the lepton pair style in a hybrid or hybrid/overlay pair style so that you use the CLASS2 LJ for the pairs of types where you use both A and B and lepton for those pairs where B is 0.
