Subject: SUMMARY: identifying single configuration systems

From: Roy Jensen <[email-masked]>
Date: Wed, 28 Aug 2002 22:51:57 -0700

Thanks to those who responded. 

ORIGINAL QUESTION
**************************************
Is it possible to tell, a priori, if a system can be reasonably and
reliably represented by a single configuration method?

Are there any systems that generally fail? transition states,
partially filled orbitals, partially filled _degenerate_ orbitals,
etc.

RESPONSES
**************************************
From: Karl Irikura <[email-masked]>

Hi.  I think you are OK as long as there are no low-lying excited 
states of the same (computational) irreducible representation.

**************************************
From: Christoph van =?iso-8859-1?Q?W=FCllen?=
<[email-masked]>

a priori is difficult, but you may run an MP2 or CISD calculation and
look at the occupation number of the natural orbitals. If they deviate
substantially from 2.0 and 0.0 (say, less than 1.9, more than 0.1)
this may be some indication for MR character.

**************************************
From: elewars <[email-masked]>

2002 Aug 23

Hello,

For a discussion of this see C. J. Cramer, "Essentials of
Computational Chemistry", Wiley, 2002, chapters 6 and 7.