[gmx-users] How to convert C6&C12 from GROMOS54a7 to sigma&epsilon for OPLSAA

[Justin Lemkul]

On 8/18/17 3:43 AM, Ming Tang wrote:
> Dear all,
>
> I want to calculate sigma&epsilon for OPLSAA.ff from C6&C12 developed based on GROMOS54a7.ff.
>
>  From the manual, I got the equations:
>
>
>
> C12 = Sigma^(6)*C6
> C6 = 4*sigma^(6)*epsilon
> However, I found sigma and epsilon are different in OPLSAA and AMBER, and that C6&C12 from GROMOS and sigma&epsilon from OPLSAA do not match these equations. Could anybody give me some guidance?

Mathematically speaking, you can certainly transform one to another, but 
you shouldn't do it.  GROMOS parameters aren't guaranteed to mix with 
any other force field, so convention wisdom is to not waste time trying 
to do it.  Force fields are self-consistent entities. Mixing and 
matching yields a physical model of unknown (likely junk) quality.

-Justin

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Justin A. Lemkul, Ph.D.
Assistant Professor
Virginia Tech Department of Biochemistry

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