[gmx-users] entries in ffG53a6nb.itp

[Erik Lindahl]

On Aug 14, 2006, at 2:49 PM, Carsten Baldauf wrote:

> dear all//
> what is the meaning of and the difference between the  
> [ nonbond_params ] and the [ pairtypes ] sections in the  
> ffG53a6nb.itp file? represents the first the normal LJ-params while  
> the latter contains the 1-4 interaction LJ-params?

Correct. Many forcefields (e.g. opls) only use scaling factors, and  
then the pairtypes section isn't necessary.

> what is the meaning of 'ptype' and 'func' in the tables?
>

ptype is particle type, which makes it possible to separate nuclei  
from bond polarization charges in polarizable force fields. In the  
standard force fields we only use atoms.

The 'func' field is an integer to specify the type of interaction,  
for instance if a bond is harmonic, cubic, or e.g. morse bond. It is  
always used even when there is only a single type of interaction  
(e.g. for position restraints), since we might add new interaction  
forms in the future.

Cheers,

Erik