From chemistry-request-: at :-ccl.net Sat May  2 00:49:18 1992
Date: Fri, 1 May 92 21:12:27 -0400
From: gt5841a {*at*} prism.gatech.EDU (ERNST,MARGOT)
Subject: virtual MOs (Doug Smith)
To: chemistry {*at*} ccl.net
Status: RO


The ordering and the coefficients of the virtual orbitals will indeed 
change for a sequence of (single determinant) HF jobs with changing 
basis sets. The virtuals resulting from such a calculation will not 
usually be representative of "good" antibonding MO's , since it is the 
occupieds that are being optimized.
In an MCSCF calculations those orbitals which are placed into the active 
space will resemble actual antibonding MO's more closely, and for a multi-
reference wavefunction there is no "ordering" of those partially occupied 
orbitals i the active space. For the purpose of interpretation, natural 
orbitals with the appropriate occupation numbers would be more suitable.
You would expect the HF orbitals to change quite a bit if you use them as 
initial guess for the MCSCF orbitals (within the valence space).
One needs to be careful in attempting to compare a MR wavefunction to 
a single determinant wavefunction, and density differences would contain 
more information (and meaning) than just the orbitals.