RE: W:Symmetry breaking during CASSCF

Jahn-Teller effect, perhaps?   Does the D2h geometry have degenerate
 orbitals which become non-degenerate once the symmetry is broken?
 --David Shobe, Ph.D., M.L.S.
 S|d-Chemie, Inc.
 phone (502) 634-7409
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 -----Original Message-----
 From: Computational Chemistry List [mailto:chemistry-request|at|ccl.net] On Behalf Of Gustavo, L.C., Moura
 Sent: Friday, July 08, 2005 7:35 AM
 To: chemistry|at|ccl.net
 Subject: CCL: W:Symmetry breaking during CASSCF
 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|at|mercury.chem.pitt.edu
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