[gmx-users] force fields & Ryckaert-Bellemans torsional potential

[dimka]

How do I convert charmm torsional FF entry with multiplicity of 6 into
Ryckaert-Bellemans torsional potential?

Gromac's RB coefficients go only up to 5, but when you expand the
Ryckaert-Bellemans dihedral potential for n=6 you get:

Vd(ijkl) = k[1+(32cos**6(x)-48cos**4(x)+18cos**2(x)-1)]

do you just drop the C6 coefficient?

the Charmm entry is:
   CTL2 CTL2 CTL2 CTL2     0.10    6   180.00 ! alkane, 4/98, adm jr.

-d