CAS-SCF active space

From: Jamie Platts <Platts^at^Cardiff.ac.uk>
Subject: CAS-SCF active space
Date: Mon, 21 Jul 2003 16:45:47 +0100 (BST)

  Hi Folks - I wonder if I can ask your advice on selecting active spaces
 for CAS-SCF calculations. I have a system which wants to form a broken-
 symmetry singlet UHF wavefunction, rather than the expected RHF one.
 The natural orbitals from this (152 e) UHF wavefunction look like:
                      71        72        73        74        75
 EIGENVALUES --     1.97076   1.95176   1.94478   1.78912   1.46209
                      76        77        78        79        80
 EIGENVALUES --     1.22142   0.77858   0.53791   0.21088   0.05522
 Can I safely ignore everything up to 73, and use a (6,6) active space
 made up of 74 to 79? Or is 1.95/0.05 too big to ignore?
   Many thanks in advance,
   Jamie
 ----------------------------------------------------------
   Jamie Platts
   Dept. of Chemistry  		Phone: +44 (0) 2920 874950
   Cardiff University 	 	Email: platts^at^cf.ac.uk
   P.O. Box 912 			FAX:   +44 (0) 2920 874030
   Cardiff CF10 3TB 		www.cf.ac.uk/chemy