[gmx-users] force fileds gromos and oplsaa

[Justin Lemkul]

On 11/16/17 8:28 AM, Faezeh Pousaneh wrote:
> Hi
>
> sorry for silly question, but I did not find my answer by searching.
>
> In gromos force fields, the file nonbonded.itp includes nonbonded
> parameters for two different atoms, e.g
>
> C O 1 2.300953E-03 2.222000E-06
>
> while I do not see oplsaa provides parameters for nonbonded interactions of
> different atoms, why?

The [nonbond_params] section specifies atom pairs for which normal 
combination rules do not apply, e.g. pair-specific Lennard-Jones 
parameters (CHARMM calls these NBFIX, for instance).

Some force fields use them, others don't (and calculate everything via 
combination rules). In the case of OPLS-AA, there are a huge number of 
atom types, more than any other force field (AFAIK), which is 
essentially doing the same thing - there are highly specific parameter 
optimizations for given interactions.

-Justin

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

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