From: AnkiReddy katha (
Date: Thu May 07 2015 - 02:12:03 CDT

Dear NAMD users

I am trying to use OPLS force field in NAMD.

vdwGeometricSigma yes
1-4 scaling 0.5

The above are changes we need to make for non-bonded interactions.

Bonded interactions (bonds term and angle term) needs no change as both
OPLS and CHARMM type used same expressions.

Dihedral term expression also is similar, so one can easily adapt OPLS
dihedral parameters into the CHARMM type.

There seems to be an issue with Improper style.

OPLS force field uses periodic improper dihedral, CHARMM type uses
harmonic improper dihedral

One idea I had was that we can have have impropers and propers under the single
head "dihedrals" in the parameter file, like OPLS style as shown below.

74 21 5 9 8 OPLS_IMPROPER 0.000000 0.000000 2.000000
  0.000000 0.000000 0.000000 0.000000
75 15 11 10 5 OPLS_IMPROPER 0.000000 0.000000 8.000000
    0.000000 0.000000 0.000000 0.000000

If we do that, then NAMD may not calculate "improper" energy term
explicitly, however it includes in the "dihedral".

Also it may consider 1-4 interactions between the atoms which are 1
and 4 atoms, which are actually not separated by three


If some of you have used OPLS in NAMD, could you please clarify how
you handled OPLS_impropers.

Thank you very much

Best Regards