Dihedral and non-bonded parameter conversion from charmm to gromacs

[Megha]

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Hi GROMACS community,

Much information is available on bond and angle coefficient conversion from GROMACS to CHARMM and vice versa, but I couldn’t find anything useful for non-bonded parameters.

For e.g. when the functional form of dihedrals is the same in both i.e., both use the following form:
K(1+cos(n*(phi) - 180))
then what is the relationship between K(GROMACS) and K(CHARMM)?

Also for non-bonded parameters, there is only conversion of units right, like for epsilon - kJ/mol to kcal/mol and otherwise; and for sigma - nm to angstrom and otherwise, right?

Thank you
Megha

[jalemkul]

@Megha

then what is the relationship between K(GROMACS) and K(CHARMM)?

It’s simply a matter of converting from kcal/mol to kJ/mol.

@Megha

Also for non-bonded parameters, there is only conversion of units right, like for epsilon - kJ/mol to kcal/mol and otherwise; and for sigma - nm to angstrom and otherwise, right?

Yes, but note that CHARMM files provide R_min/2, not σ so you have to do that additional conversion.