[gmx-users] OPLS-AA to CHARMM conversion in gromacs ffnonbonded.itp

[tarak karmakar]

Dear All,

I have a little confusion with the non-bonding parameters conversion from
OPLS-AA to CHARMM in gromacs.
If I see the ffnonbonded.itp in both the cases I get the following numbers

OPLS-GROMACS

Sigma = 0.1644471 nm
Epsilon = (0.875044*4.184) = 3.66118 kJ/mol

.................................................................................................................................................................
CHARMM-GROMACS

Sigma = 0.21114299 nm
Epsilon = 0.06276 kJ/mol

Now, from the charmm27 parameters file I get
CHARMM27

Rmin/2 = 1.18500 Ang
Epsilon = -0.0150 kcal/mol

Converting these charmm parameters to gromacs formate I have to do
following conversions

Sigma = 2^(5/6) * (Rmin/2) = 2^(5/6) * 1.185 = 2.1114299 Ang = 0.2114299 nm
Epsilon = 0.0150 * 4.184 = 0.06276 kJ/mol
....................................................................................................................................................................

So, CHARMM (par_water_ions.prm) to charmm(ffnonbonded.itp) is fine. But the
problem I'm facing to convert the OPLS(ffnonbonded.itp) to
CHARMM(ffnonbonded.itp).

I apologize for making any confusion.


Thanks,
Tarak