[gmx-users] Files for CHARMM 27 force field

[Mark Abraham]

Shaul Wallach wrote:
> Last October, Dr. Yuguang Mu reported that he uploaded files for the CHARMM
> 27
> force field to the GROMACS site at
> 
> http://www.gromacs.org/topologies/force_fields.php

I used to use these forcefields and wasn't 100% satisfied they were 
implementing CHARMM rigorously. In particular, Urey-Bradley terms were 
implemented with a combination of angle terms and position restraint 
terms that required you to actually know to turn position restraints on, 
and there was something else I thought was wrong but can't remember now 
what it was. Maybe it was the way the dihedral terms were being managed 
- there needed to be some serious massaging because of the different 
functional forms or something. Part of the issue with the U-B terms was 
that pre-3.3.1, attempting to use the U-B code generated a fatal error, 
which DvdS and I discussed on this list back in Jan/Feb this year - this 
problem may have motivated the hack solution described above.

Anyway, to make sure that things were being done properly I actually 
wrote a Perl script that converted the CHARMMxx file format into the 
formats necessary for GROMACS. Thus people can satisfy themselves that 
it is/was being done properly. I'll knock the cobwebs off that and 
contribute it in the next few days.

Mark