CCL:G: Relaxed PES in g03 using forcefield?

[Ben King <king]![chem.unr.edu>]

 Sent to CCL by: Ben King [king^chem.unr.edu]
 Dear CCLers:
Summary:  how can one use gaussian to perform a relaxed potential
 energy scan with a force field?
We need to generate a 2-D potential energy surface for a bond-
 breaking coupled with a torsion.  Gaussian's tools for performing a
 relaxed PES using the modredundant option to opt are ideal, but Z-
 matrix tools could also work.  Here is the problem:  Gaussian (g03)
 completely ignores any constraints when we use molecular mechanics
 (amber, uff, etc.), using either Z-matrices or opt=modredundant.
 This also happens in OMION methods using a MM fragment.  Our syntax
 is correct, as we do these same calculations (albeit inaccurately and
 only over limited portions of the PES) using semi-empirical methods.
We must use a molecular mechanics force field: van der Waals
 interaction are important in our molecule and we need to study the
 portion of the bond dissociation region that is poorly described by
 single references wavefunctions (e.g., C-C bond distances of 1.4 A to
 6.0 A in 0.2 A increments). The molecule is large (C62H38), so high-
 level ab initio methods are prohibitively time consuming.  Molecular
 mechanics will give us the most realistic description of the
 molecular behavior, provided that we use a Morse potential for the
 bond dissociation.
If anyone can suggest a procedure to generate a PES from a force
 field, I would be grateful. This is one of those irritating things
 that should be easy but ends up being exceptionally difficult.
 Also,  if anyone knows why Gaussian behaves so oddly with MM methods,
 I would be delighted to know.
 Thanks,
 Ben
p.s.  The version of Gaussian is: Gaussian 03:  AM64L-G03RevC.02 12-
 Jun-2004
 p.p.s.  We are limping through this by running thousands of
 individual constrained optimizations using Tinker, but it isn't
 pretty...
 ----
 Benjamin T. King, Associate Professor
 Department of Chemistry/216
 University of Nevada, Reno 89557
 tel (775) 784-1736 fax (775) 784-6804
 king-.-chem.unr.edu
 http://www.chem.unr.edu/faculty/btk/