Subject: Fudge Factors and Computational Shortcuts

From: [email-masked] (Joe Golab)
Date: Fri, 25 Oct 1996 16:12:12 -0500

We all know about the "famous" Hartree-Fock vibrational frequency fudge factor
of 0.89 for calculated values.  This came about from a lot of work (and
observation) on simple and not so simple molecules.  See Hehre, Radom,
Schleyer, and Pople, "Ab Initio Molecular Orbital Theory", for example.

I am wondering if there are similar "famous" fudge factors, specifically for
electronic energies.  For example, how much of the correlation energy does an
MP2 single point calculation (at the converged HF stationary point) obtain
versus the fully optimized MP2 final energy?  50%?  60%?  I'm sure that for
some systems there is no simple correlation since the HF description could be
very poor (like O2).  But does any literature exist on this subject?  Word of
mouth?  Anecdotal?

On a similar subject, is there literature (etc) on how good (if at all)
hessians used in optimizations, e.g. updated, are for the calculation of
the optimized structure's vibrational frequencies?

Both of these questions are directed at a search for fast and quasi-reliable
ESTIMATES of what a full calculation would predict.  For example, at
convergence, can the updated hessian resident in memory or on disk at that
time be used to predict the vibrational frequencies of the stationary point
(for thermochemical data) immediately?  Obviously, the hessian would need to
be calculated at the stationary point to get "real" answers but in the
meantime, could the updated hessian's spectrum be used (for ESTIMATES) while
that (longer than a diagonalization) calculation progresses?

Comments will be summarized and posted in due time.

-- 

:Joe
 [email-masked]
 (630) 961-7878  <SOCON 8 231 7878>

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