ERG group members work with different types of molecules and consequently added new atomtypes into classical OPLS-AA distributed with GROMACS. Now the ERG patch of OPLS-AA force field is published at bitbucket as a repository.
The repository now is hosted at bitbucket: http://bitbucket.org/comcon1/oplsaa-erg_ff.
To use our force field, just sync the repo:
hg clone http://bitbucket.org/comcon1/oplsaa-erg_ff cur.ff
Next, you can simply use pdb2gmx or grompp from this directory, selecting cur.ff as a default force field.
If you use our tool TPPMKTOP, you should also synchronize the repository.
For developers
Locally we work with the ERG mercurial version of the force field: http://erg.biophys.msu.ru/hg/oplsaa-erg_ff
Local private repository is synchronized to the bitbucket repository after crucial modifications are comitted.