[gmx-users] Conversion CHARMM FF to Gromos54a7 FF

[Justin Lemkul]

On 1/14/19 11:28 AM, Salman Zarrini wrote:
> 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).

You cannot convert one force field to another. A force field is a 
self-consistent, balanced entity. Even if you could (mathematically 
speaking) turn one form of parameters into another, the results would be 
physically meaningless.

-Justin

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Justin A. Lemkul, Ph.D.
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