Subject: Small +ve freqs in HF

From: Jeremy R Greenwood <[email-masked]>
Date: Fri, 1 Dec 1995 14:03:26 +1100 (EST)

Greetings all,

Some simple questions for those more familiar with ab initio than I:

As I understand it, the presence of a small real frequency in a
Hartree-Fock calculation is regarded as evidence that on increasing
basis size, the constrained structure may turn out to be a saddle point, 
with the mode in question becomming imaginary.

My question is: How small is small?

I am performing calculations on semi-aromatic heterocycles, and at
HF|6-31g* in Cs symmetry, I find a frequency of 8 /cm. Others around 
30-50 /cm. The ring is somewhat prone to puckering, and indeed some 
similar calculations have produced C1 structures.

My other question is: Does inclusion of electron correlation (e.g. MP2) 
with analytic second derivatives tend to produce similar results in terms 
of symmetry breaking as increasing basis set? (I understand that answers 
to these questions may depend very much on the nature of the systems 
being studied.)

Jeremy
--
Jeremy Greenwood
Department of Pharmacology
University of Sydney