This repository contains scripts and codes which enable the use of some non-standard interaction potentials with molecular simulation software, mainly gromacs and lammps.
The Weeks-Chandler-Anderson (WCA) model potential is a purely repulsive potential used to model excluded volume interactions. The potential is constructed by truncating the Lennard-Jones potential at r = 2^{1/6} \sigma and shifting it by \epsilon ensuring that the potential decays smoothly to 0 at r = 2^{1/6} \sigma. This scheme can be used more generally to scale the attractive portion of the interaction potential without much modification to the repulsive portion. A scaling value of 1.0 results in the normal Lennard-Jones potential. The scaled potential is given by:
4*\epsilon*[ (sig/r)**12 - (sig/r)**6 + (1 - \lambda)/4 ] , r< 2**(1/6) \sigma
u(r) =
4*\epsilon*\lambda*[ (sig/r)**12 - (sig/r)**6 ] , r >= 2**(1/6) \sigma
The gromacs tabulated potential option provides an easy way to use the WCA scaled potential. The table-gmx.f90/table-gmx.py codes produce interaction tables for use with gromacs.
For use with lammps, we have implemented new pair_style's based on pair_style lj/cut/coul/long and pair_style lj/cut. The potential can be used with the following syntax:
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style name: lj/cut/coul/long/wca
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options: pair_style lj/cut/coul/long/wca <cut_lj> <cut_coulomb>
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coefficients: pair_coeff <cut_lj>
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style name: lj/cut/wca
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options: pair_style lj/cut/wca <cut_lj>
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coefficients: pair_coeff <cut_lj_one>