Subject: Re: CCL:validity of constrained calcs.

From: [email-masked] (Joel Polowin)
Date: Mon, 17 Jul 95 10:36:22 -0400

> Date: Thu, 13 Jul 1995 13:24:12 -0700 (MST)
> From: Matt Stahl <[email-masked]>
> Subject: CCL:validity of constrained calcs.
> 
> 	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. 

If you add a constraint -- say, a torsion angle -- then none of those atoms
have their normal freedom to move; they and their attachments won't be
fully optimized.  Through non-bonding interactions and propagated bonding
interactions, everything else will be slightly distorted as well.  If this
is what you're talking about, it's a problem inherent in the procedure.  In 
my opinion, it is reasonable to use these results as long as one keeps that 
problem in mind; in general I think that the structural distortions are
not serious.

In principle, I suppose it would be possible to get the same results in
other ways.  Most of my work has involved incrementing a constrained
torsion angle to get the barrier to rotation of part of a system with
respect to the rest.  Instead of doing that, I imagine that one could
set up a molecular-dynamics scheme where part of the system was stationary
and the rest was started with initial velocities applied to the atoms
corresponding to the desired rotation.  For sufficiently high velocities,
the barrier would be overcome.  It would be a matter of repeating the
calculations several times with different starting velocities to find out
how much "rotational energy" would be necessary to overcome the barrier.
I'm not sure how effective such calculations would be compared to the
usual procedures.

Joel
[email-masked]

 	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
> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> 
> 
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