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Date: Sat, 27 Sep 1997 09:07:05 -0400
To: chemistry@www.ccl.net
From: Dawn Friedman <dsf@world.std.com>
Subject: G:G94 RIC and GAMESS: constrain var to equivalence?





  This is only partly a G94 question -- my real concern is with GAMESS.

  I'm geometry-optimizing some model systems whose high symmetry is broken
only by methyl rotors.  I want the methyl hydrogens to be free to get out of
each other's way, but I want the C-C frame bonds to stay equivalent, at
least until I have a partial optimization.  

  This is easy to do in a Gaussian z-matrix format.  G94 can be forced to
use the input zmatrix rather than redundant internal coordinates, though of
course all the advantages of the RICs are lost.  (Is there a mathematical
reason why G94 can't construct an RIC set with some variables constrained to
be equal?)

  I haven't figured out *any* way to constrain a set of bond lengths or
angles to be equal *but variable* in GAMESS when the entire molecule doesn't
have symmetry.  My choices seem to be:

  (a)  Force the entire molecule, including all the methyl hydrogens, to
exhibit at least one symmetry element.

  (b)  Add fixed constraints in $STATPT, which will hold the C-C bond
lengths to a constant value, and hope I picked the right one :-)

  I'm working with model systems that aren't necessarily stable as single
molecules in a full optimization, so I can't count on their optimizing to an
energy minimum if I don't constrain them.  Should I just choose option (a)
and let the CH3 hydrogens suffer?  Is the hybrid I'm trying to create --
symmetric frame and free methyl hydrogens -- inherently invalid?  I'll
summarize to the list.

  Thanks,
  Dawn Friedman
 

  
     
  Dawn Friedman  dsf@world.std.com