Subject: wavefunction stability

From: Dale Andrew Braden <[email-masked]>
Date: Wed, 11 Jun 1997 16:51:28 -0700 (PDT)

Dear CCL,

I have just come across an old article (Theor. Chim. Acta 48 (1978) 103) 
in which the authors indicate that RHF-->UHF wavefunction instabilities
are quite common.  From reading the literature, it seems that people
rarely check HF wavefunctions for stability.  What I am wondering is how
seriously calculated molecular properties-- particularly vibrational
frequencies-- are affected by the use of unstable HF wavefunctions. 
Likewise, Moller-Plesset results are also questionable.  And even if one
can find a UHF solution for a closed-shell system, at the HF/MP level this
often introduces a high degree of spin contamination, which should render
the calculated molecular properties equally unreliable. 

What thoughts do people have on this matter?

Best regards, 

Dale Braden
Department of Chemistry
University of Oregon
Eugene, OR 97403-1253
[email-masked]