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

[Justin Lemkul]

On 5/31/13 1:18 PM, tarak karmakar 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).
>

Why should they be the same?  They're different force fields, so they have 
different parameters.

-Justin

-- 
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Justin A. Lemkul, Ph.D.
Research Scientist
Department of Biochemistry
Virginia Tech
Blacksburg, VA
jalemkul[at]vt.edu | (540) 231-9080
http://www.bevanlab.biochem.vt.edu/Pages/Personal/justin

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