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

[tarak karmakar]

All these parameters are for Mg2+, forgot to mention.



On Fri, May 31, 2013 at 10:48 PM, tarak karmakar <tarak20489 at gmail.com>wrote:

> 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
>