RHF, UHF, CASSCF and CASPT2 calculations for (cyclobutadiene)tricarbonyliron, CbFe(CO)(3)

The energy of CbFe(CO)(3) was calculated with RHF and CAS(12,12) methods as a function of Fe-Cb distance. RHF wave function was shown to be unstable i n a wide range of Fe-Cb distances. Two types of UHF solutions were found, o ne which breaks the symmetry of the Fe-Cb bond, and another where the symme try breaking in the ring also occurs. The symmetry breaking in Fe-carbonyl bonds was found to be important, too. The geometry of CbFe(CO)(3) was optim ized with RHF and CAS(10,10) methods, while the Fe-Cb and Fe-CO distances w ere optimized also at the CASPT2 level. CAS(10,10) optimized Fe-C(Cb) bond lengths reproduce correctly the experimental trends while the RHF optimized geometry was found to be in disagreement with experiment. The rotational b arrier calculated is equal to zero in all methods. The Fe(CO)(3)-Cb binding energy was calculated on CASPT2 level. (C) 1999 Elsevier Science B.V. All rights reserved.