CHARMM Development Project Forums User Discussion & Questions Parameter Set Discussion trouble using OPLS combining rules

Dear CHARMMers,

I'm having trouble with the GEOM keyword to use OPLS combining rules. I have tried three versions (c29b1, c32b1, and c33a1) which were compiled with OPLS in pref.dat (show -> OPLS=1), yet all fail to recognize the GEOM keyword. Only version 29b1 gives an error (extraneous characters) when reading GEOM in the parameter file, but all three give the same error when reading this option later in the input file. The VDW energy is unchanged from the default nonbond options for CHARMM parameters - it is still using arithmetic combining rules.

I noticed a difference in how the keyword is invoked between the OPLS parameters distributed with charmm {charmm_dir/toppar/non_charmm/par_opls_aa.inp}
NONBONDED nbxmod 5 atom cdiel switch vatom vdistance vswitch -
cutnb 14.0 ctofnb 12.0 ctonnb 11.5 eps 1.0 e14fac 0.5 wmin 1.5 -
geom !assigns use of OPLS combining rules

and the parameters from C. Brooks' website http://brooks.scripps.edu/charmm_docs/Data/oplsaa-toppar.tgz
NONBONDED nbxmod 5 atom cdiel switch vatom vdistance vswitch -
cutnb 14.0 ctofnb 12.0 ctonnb 11.5 eps 1.0 e14fac 0.5 wmin 1.5
COMB GEOM

The COMB GEOM command was not recognized either. What am I missing?

Thank you,

james

The GEOM keyword is only recognized in the parameter file, and should not be used in other non-bond specifications, eg in your main input file. I also believe that it is only recognized if you use a CHARMM executable with the IMCUBES feature included; use command PREF to see if this is the case. The COMB GEOM is variant is an obsolete construct.

Thank you for the clarification Lennart. It is behaving as expected when I use/don't use GEOM in the parameter file, but does not change when I specify GEOM/ARIT in the input file. Looking at parmio.src, you are correct about requiring IMCUBES to use the GEOM rules. Thank you again!

james