W:Symmetry breaking during CASSCF

From: "Gustavo, L.C., Moura" <gustavo _()_ mercury.chem.pitt.edu>
Subject: W:Symmetry breaking during CASSCF
Date: Fri, 8 Jul 2005 07:34:44 -0400

Dear CCL Readers,
      I have a planar conjugated organic molecule that, on paper, has D2h
 symmetry. I am working with the hypothesis that the molecule is a
 biradical singlet. I want to perform a CASSCF(2,2) on this molecule
 to estimate the biradical character of this molecule. When I
 calculate the molecule with the constraint that the molecule has D2h
 symmetry, the calculation has severe convergence problems. If I
 remove this constraint, the molecule distorts to a C2v symmetry and I
 obtain a biradical character ~20%. I checked the orbitals being used
 for the CASSCF calculation and they look right (one nonbonding and
 another slightly antibonding).
      Is the situation described above correct? Can an organic molecule
 have such a symmetry breaking as a result of a CASSCF calculation? Is
 the molecule really a C2v system? Any suggestions are welcome.
      Thank you very much in advance for your help.
      Sincerely yours,
                                 Gustavo L.C. Moura
                            gustavo _()_ mercury.chem.pitt.edu