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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