Subject: Vibrational analysis w/ constraints

From: "Jan H. Jensen" <[email-masked]>
Date: Fri, 11 Aug 2000 16:56:02 -0500

Hi,
	I am interested in doing a vibrational analysis of a Hessian matrix
calculated for a system that was optimized with geometric constraints.  Can
anyone point me to literature of this subject?

	Thanks, Jan Jensen

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Jan H. Jensen				Assistant Professor
Department of Chemistry			[email-masked]
University of Iowa			Phone:(319) 335-1108
Iowa City, IA 52242			FAX:  (319) 335-1270
http://www.uiowa.edu/~chemdept/faculty/jensen/
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From: TREVOR D POWER <[email-masked]>
Date: Fri, 11 Aug 2000 18:00:12 -0500 (CDT)

Jan,
	If you mean you want to perform a second-derivative vibrational
frequency analysis on a geometry obtained from a partial geometry
optimization, I have a couple of responses.  To my knowledge there is no
way to do such a thing with Gaussian.  But, more importantly, why would
you want to do such a thing?  A vibrational analysis at some point, other
than an extrema, is totally meaningless.  The stationary point obtained
from a partial geometry optimization is not a true stationary point at
all.  

Cheers,
David Power
Department of Chemistry
University of North Texas
NT Station, Box 305070
Denton, TX 76203-5070
[email-masked]

On Fri, 11 Aug 2000, Jan H. Jensen wrote:

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From: "T. Daniel Crawford" <[email-masked]>
Date: Fri, 11 Aug 2000 22:17:38 -0400 (EDT)

A superb reference for computing force constants at constrained
geometries is:

"On the ab initio determination of higher-order force-constants at
non-stationary reference geometries," W.D. Allen and A.G. Csaszar, 
J. Chem. Phys. /98/, 2983-3015 (1993).

When the forces are not zero, one must project out rotational components
of the quadratic force constants and take care in selecting internal
coordinates.  There are definitely good reasons to do such an analysis;
Allen and Csaszar found that one can often obtain CCSD(T)-quality results
simply by using high-accuracy experimental geometries as references with SCF
frequencies, for example.

-Daniel

--
T. Daniel Crawford                           Department of Chemistry
[email-masked]                          Virginia Polytechnic Institute and
www.chem.vt.edu/chem-dept/crawford              State University