[gmx-users] question about modified LJ combination rule

[sujithkakkat .]

Dear all,

    I am studying a water-methane system with tip4p-Ice for water and
oplsaa for methane.

What I want to try is modify the intermolecular non-bonded interaction
between methane and water. I just want to multiply the combination rule
(geometric mean) used to obtain the  mean well depth (epsilon) component by
a scaling factor, without changing any of the non bonding function types.

 I am aware that user defined non-bonding functions are possible. However,
here I don't want to change the function type, but only slightly modify the
combination rule 3 , by multiplying the epsilon part  with a scaling factor.

Also, I find that there is no nonbond_params section in OPLSAA where I
think it would have been possible to edit cross LJ terms .

So, is there an easy way I can do it rather than going for user defined
non-bonded functions .

Is it ok to  define "nonbond_params" in topology file with oplsaa
forcefield?

Please comment.

Thanks,

Sujith.