Subject: Summary:wave function stability question

From: "Dale A. Braden" <[email-masked]>
Date: Wed, 11 Nov 1998 07:59:10 -0800 (PST)

Dear CCL,

Thank you all for your rapid and thorough answers to my questions.  Below
is a summary of the answers.

 1) Am I correct that [a wave function] stability calculation is NOT
    guaranteed to recognize an excited state?
 
Yes.  The wave function stability calculation checks single excitations
only, so a doubly-excited state would not be recognized.  However, it
appears that converging to a doubly excited state would be most unlikely,
so that performing a stability calculation is in fact a very good check.

 2) Are negative eigenvalues for virtual orbitals a SURE sign that an
    excited state has been obtained?

No, not at all for DFT, and this is in stark contrast to the situation for
a HF calculation.  The interpretation of a negative virtual orbital energy
in DFT is that an electron would be bound if it occupied that orbital.
 
 3) In case I really still do have an excited state wave function, is the
    only way to direct it towards the ground state to guess which orbitals
    should be occupied, and then use guess=(read,alter) to fix their
    occupations?  (I'd like to avoid hit-or-miss strategies!)

If you suspect that you really have an excited state, then performing a
stability=opt calculation is probably the best strategy.  Another way, but
perhaps less certain, is to feed the wave function into a different ab
initio package which is able to use the same method, basis set, etc.  If
the energy drops significantly, then it was an excited state.  But if a
stability=opt calculation reports that the wfn is stable, and you STILL
think it's actually an excited state, then you'd have to do a CISD
calculation to see whether double-excitations make a difference.

Best wishes to all,

Dale

Dale Braden
Dept. of Chemistry
University of Oregon
Eugene, OR 97403-1253