[gmx-users] using two different combination rules for the LJ potentia

[Mark Abraham]

Ana West wrote:
> Thank you so much for the kind reply. I made a mistake in my previous 
> posting - I am dealing with combination rule 1 and the combination rule 
> 2. Initially I converted the parameters according to section 5.3.3 of 
> the manual and when I did some tests to verify my force-field, the 
> minimum of the potential well seemed fine but the repulsive part of the 
> potential was not steep enough. The conversion did not seem to work for me.

Why is this problem of mixing combination rules even arising?

> Is there sort of a qualitative way of knowing when the conversion from 
> section 5.3.3 fails? (like maybe it fails when one has relatively high 
> partial charges ?!?1). If I would know that then maybe I will get more 
> lucky with the conversions for the other molecule and keep the 
> combination rules for the one for which I know the conversion parameters 
> fail.

The charges are irrelevant - these parameters are modelling vdW 
interactions.

Mark