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

Peter C. Lai pcl at uab.edu
Wed Apr 6 13:40:58 CEST 2011

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
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,

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)
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!
Peter C. Lai                 | University of Alabama-Birmingham
Programmer/Analyst           | BEC 257
Genetics, Div. of Research   | 1150 10th Avenue South
pcl at uab.edu                  | Birmingham AL 35294-4461
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