[gmx-users] GROMACS OPLS-AA question

[Eric Smoll]

Hello GROMACS users,

I am interested in using the OPLS-AA as implemented in GROMACS.
The OPLS-AA forcefield omits all nonbonded (Coulomb and LJ) interactions 1
and 2 bonds away. Nonbonded interactions 3 bonds are away are scaled by
1/2. Nonbonded interactions are applied without modification for atoms 4 or
more bonds away or between atoms in different molecules.

In GROMACS, this is accomplished by excluding Coulomb and LJ interactions
1, 2, and 3 bonds away with the following directive:

[ moleculetype ]
; name nrexcl
System 3

The scaled intramolecular interaction at 3 bonds away is added in
separately as a "pair" interaction. This is either done manually with "[
pairtypes ]" and "[ pairs ]"  directives or automatically by including
"yes" as the third entry under the "[ defaults ]" directive.

*Is this correct?*

The manual states that "the use of [Ryckaert-Bellemans potential functions]
implies exclusion between the first and last atom of the dihedral." *This
statement is referring to the use of  nrexcl=3 in the "[ moleculetype ]"
directive as shown above, correct? The use of RB dihedral functions in
GROMACS does not imply that the scaled "pair" interactions are not applied,
correct?*

Best,
Eric