Subject: validity of constrained calcs.

From: Matt Stahl <[email-masked]>
Date: Thu, 13 Jul 1995 13:24:12 -0700 (MST)

Netters,
	I am working on a project which involves parameterizing molecular
mechanics torsion potentials.  We used gaussian to generate energy vs. 
torsion data by performing constrained optimizations.  One torsion angle was
held constant while the rest of the coordinates in the molecule were
optimized.  Someone recently made the comment that constrained optimizations
with gaussian are not appropriate for such a study because the 
unconstrained coordinates do not fully optimize.  This suggestion seems 
odd because the literature seems to be repleat with examples of isolating 
the energetics of a molecular motions (distances, angles, and torsions) 
through constrained optimization.  How far off the mark am i in thinking 
that constrained opts. are appropriate for this type of work. 
	The alternative method would be to perform an IRC (internal reaction
coordinate) calculation to characterize the path between the two minima of
interest, but as i understand it IRC calculations are dependant on Linear
Synchronous Transit calculations which ESTIMATE the transition state.  Can
one perform an IRC from a calculated transition state (opt=ts)?  If not, how
much more "valid" is an IRC calculation than constrained optimization if at
any point the IRC has to "guess" the path? 

thanks,
matt

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Mr. Matthew Stahl			| 
Department of Chemistry			| I'd rather be right half of the
University of Arizona			| time than half right all the
Tucson, AZ  85721			| time.
[email-masked]		|	-Malcom S. Forbes
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~