Subject: QCISD energy lower than QCISD(T) energy!

From: [email-masked] (Anthony P. Scott)
Date: Wed, 22 Apr 1998 09:45:12 +1000

We recently have such a case. The molecule in question is CaO. Both G94,
molpro96 and ACES2 agree on this, although the molpro results are different
from the G94 and ACES2. (we are investigating this still).

The basis used is 6-311+G(3df,2p).

When the 6-311G** or 6-311+G** basis is used the QCISD energy is lower than
the QCISD(T) energy as I would normally have expected.

Can someone shed some light on this strange result. Is such a result even
physically meaningful? and if so what exactly does it mean?

A possible reason is linear dependency in the large basis set, however for
BeO and Ca(OH)2 such a strange thing does not happen.

Thanks for your help.

tony

______________________________________________________________________________
Dr. Anthony P. Scott,
Computational Quantum Chemistry Group,   Office Ph.: 61-2-6249-3573
Research School of Chemistry,            Dept. Ph.:  61-2-6249-3637
Australian National University,          Fax:        61-2-6249-0750
Canberra, ACT 0200,                      E-mail:     [email-masked]
AUSTRALIA.
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