[gmx-users] Mixing Force Fields

Mark Abraham Mark.Abraham at anu.edu.au
Fri Oct 14 17:38:53 CEST 2011

On 15/10/2011 2:23 AM, Fabian Casteblanco wrote:
> Hello community,
> I have a general question about mixing force fields.  I want to try to
> use CGenFF to model the drug and I wanted to use CHARMM for the
> solvent.  I see the general advice given online is to always use only
> one forcefield but I know CGenFF was developed off CHARMM but
> specifically for drugs.  If anything, I would like to try it to see
> how different results are from just using CGenFF.  How would one go
> about using two forcefields on your topology? For example below, would
> one simply add the non-default FF to the default FF?
> ;	File 'topol.top' was generated
> ;	By user: Administrator (500)
> ;	On host: fabian-3bef2d6a
> ;	At date: Fri April 1st 02:59 PM 2011
> ;
> ;	This is your topology file
> ;	DRUG SOLUTION w/ Ethanol
> ; Include forcefield parameters
> #include "Charmm.ff"
> #include "Cgenff.ff"
> **Thanks for everyones help.

It won't Just Work because you cannot have two [defaults] sections. 
There may be other such issues. Reading chapter 5 of the manual will be 
key here.


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