open-shell force fields

[Thomas.Bally #*at*# unifr.ch (Thomas Bally)]

 Dear Netters,
   we are attempting to fit force fields of open-shell species (triplet
 carbenes, radicals, radical ions) starting from Hessians computed by
 some quantum chemical procedure and we face the problem that UHF and
 ROHF often give *very* different valence force fields (in spite of the
 fact that the geometries often do not differ that much). In cases of
 large <S**2> values, we wish to avoid UMP2 and the available R(O)MP2
 codes (Cadpac, Aces) often give us nonsensical results.
   Is anyone aware of any systematic studies which have been conducted
 to assess the reliability of ROHF vs. UHF (or any other methods such
 as DFT) for force fields or vibrational frequencies of open-shell species?
   Please reply to my personal e-mail address, I will post a summary to
 the net if I get any useful hints.
   Thanks for your help!
   thomas
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 |  Prof. Thomas Bally                 |  E-mail:  Thomas.Bally #*at*# unifr.ch
 |
 |  Institute for Physical Chemistry   |                                   |
 |  University of Fribourg             |                                   |
 |  Perolles                           |  Tel:     011-41-37 826 489       |
 |  CH-1700 FRIBOURG                   |  FAX:     011-41-37 826 488       |
 |  Switzerland                        |                                   |
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