[gmx-users] Conversion CHARMM FF to Gromos54a7 FF

[Salman Zarrini]

Dear all,

I have a CHARMM all atoms force fields for a solid with limited atom types,
and I want to convert them to Gromos54a7 force fields. Would you please
confirm that below are the changes I should make to do so?

1. Combination rule # 2 is used in CHARMM whereas GROMOS which uses # 1,
so, In  [ atomtypes ] section I should convert the \sigma and \epsilon
defined in CHARMM to V(c6) and W(c12) in Gromos, respectively to be used in
Gromos. Where, the V(c6) = 4*\epsilon*(\sigma)**6 and W(c12) =
4*\epsilon*(\sigma)**12? (page 123-124 Gromacs.18 manual)

2. In [ bond ] section, the bond function should be 2 (in Gromos) vs 1 in
CHARMM. (Table 5.5, page 139, Gromacs.18 manual)

3. in [ angle ] section, the angle function should be 2 (in Gromos) vs 5 in
CHARMM. (Table 5.5, page 139, Gromacs.18 manual).

Thanks.
Salman