[gmx-users] modify C6 directly in FF using sigma and epsilon

[David Mobley]

Gromacs users,

I'm using a force field (the port of AMBER to GROMACS) which uses
combination rules (combination rule 2), etc. in terms of sigma and
epsilon. However, I need to be able to modify C6 directly (in
particular, I want to be able to set it to zero). Can anyone give me
any pointers on a relatively straightforward way to do that? I want to
be able to continue using combination rule 2 but be able to separately
modify C6 for a couple atom types (or interacting atom pairs).

Obviously, one way I could work around this would be to explicitly
specify all of the nonbonded interaction pairs in my topology so that
the combination rules aren't used, then I could change the combination
rule so that I could modify C6 and C12. But since that would be a real
pain I'm hoping someone here can suggest something easier.

Thanks,
David