Subject: Orbital order in MCSCF

From: "Slawomir Z. Janicki" <[email-masked]>
Date: Mon, 23 Apr 2001 21:32:13 -0500

I have some problems with ordering of orbitals in MCSCF active space. I am
working on a series of compounds with seven pi orbitals in the active space.
One atom of which the pz orbital is a part of the pi system is changed to
heteroatoms in the series. The number of electrons is preserved by adjusting
the overall charge. The symmetry group is C2v.

In most cases the pi orbitals are ordered by their occupancy as you would
expect:

1 one nodal plane (B1)
2 two nodal planes (B1)
3 two nodal planes (A2)
4 three nodal planes (SOMO1) (B1)
5 four nodal planes (SOMO2) (A2)
6 five nodal planes (B1)
7 six nodal planes (A2)

In two compounds out of ten the orbitals are swapped. In one case the
orbitals are ordered 2,3,4,1,5,6,7 in the other it is 2,3,4,5,1,6,7 (the
state is always 3B2). I tried to rearrange orbitals and use more precise
integrals, but the final result is always the same. What could cause this
problem and is there a remedy? Would this change affect the total energy?

Slawomir Janicki
[email-masked]