[gmx-users] Invalid order for directive defaults

[Justin Lemkul]

On 12/10/15 10:49 AM, Daniele Veclani wrote:
> Dear Gromacs Users
>
> I'm trying to study a molecule (my solute) in methanol as solvent.
>
> For my solute I use Charmm27 ff, while for methanol I want to use opls/aa
> ff.
>
> I do this beacuse methanol is already implemented in opls force field.
>
> My .top file is:
> ....
> #include "charmm27.ff/forcefield.itp"
>
> #include "charmm27.ff/nor-antib.itp"
>
> #include "oplsaa.ff/forcefield.itp"
>
> #include "oplsaa.ff/methanol.itp"
>
> [ system ]
> ; Name
> nor+met
>
> [ molecules ]
> ; Compound        #mols
> MET              1000
> nor-antib        1
>
> But when I star groomp program I get this error:
> "Fatal error:
> Syntax error - File forcefield.itp, line 18Last line read:'[ defaults ]'
> Invalid order for directive defaults
> "
>
>
> I looked at the manual and in website at
> http://www.gromacs.org/Documentation/Errors.
> I can't fix the problem.
>
> Could you help me?
>

You're mixing two force fields.  The [ defaults ] directive provides the 
instructions for the functional form.  You introduce those instructions for 
CHARMM, then try to introduce them again for OPLS-AA.  This is nonsensical.

CHARMM has methanol already; use it rather than trying to mix force fields. 
Even if you can overcome the technical aspects of doing this (which require 
substantial modifications of force field files), your model physics is 
questionable, at best.

-Justin

-- 
==================================================

Justin A. Lemkul, Ph.D.
Ruth L. Kirschstein NRSA Postdoctoral Fellow

Department of Pharmaceutical Sciences
School of Pharmacy
Health Sciences Facility II, Room 629
University of Maryland, Baltimore
20 Penn St.
Baltimore, MD 21201

jalemkul at outerbanks.umaryland.edu | (410) 706-7441
http://mackerell.umaryland.edu/~jalemkul

==================================================