[gmx-users] help on converting charmm/cgenff parameters to gromacs

[Peter C. Lai]

On 2011-04-06 01:49:50AM -0500, Mark Abraham wrote:
> The standard CHARMM .prm files give an indication of how the parameters 
> will be used, so it's just a matter of converting units and taking care 
> of any constants.

Looks like if I use
http://lists.gromacs.org/pipermail/gmx-users/2010-September/054302.html
for sigma/epsilon, I'll need to divide the converted sigma by 10 now...

Looks like for eps1,4 I take the eps1,4 of each atom in the pair,
sqrt(charmm(eps1,4i)*charmm(eps1,4j))*4.184

And for the sigma1,4 term I take the sum of the Rmin/2, divide by 2 
then convert to sigma per pair.

For the dihedrals, it looks like 
gromacs(phi0) = charmm(delta)
and
gromacs(cp) = charmm(Kchi)*4.184

And for impropers
gromacs(cp) = charmm(Kpsi)*4.184*2

At least those look like the calculations done for the c36ff conversion.

Thanks for your help!
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Peter C. Lai                 | University of Alabama-Birmingham
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