Subject: CASSCF/CASPT2 question

From: Wibke Sudholt <[email-masked]>
Date: Mon, 8 Sep 1997 14:10:20 +0200 (MET DST)

Dear CCLers,

perhaps someone of you can help me with this problem:

I am doing excited state CASPT2 calculations with a CASSCF reference on
a medium size organic molecule. Sometimes different starting orbitals
lead to different CASSCF results probably because of several wavefunction
minima. I suppose that this can be caused by an unsufficient active space,
but I would run out of time and computational resources if I enlarge it.
I also do not want to use state average calculations. So I need a systematic
criterion to choose safely the "right" results. I am thinking of the lowest
CASSCF energy (variation principle) or the highest weight of the CASSCF
reference in the CASPT2 calculation (lowest perturbation). What would you
prefer? Or are there other criterions? Can I compare calculations with
different active spaces in the same way? I hope this question is not too
stupid, but the subject is still relatively new to me.

Any suggestions or pointers to the literature would be welcome. I will try
to summerize. Thank you very much in advance

Dipl.-Chem. Wibke Sudholt
Institute of Theoretical Chemistry
Heinrich-Heine-University Duesseldorf
Germany
[email-masked]