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

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 in a wide range of Fe–Cb distances. Two types of UHF solutions were found, one which breaks the symmetry of the Fe–Cb bond, and another where the symmetry 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 optimized with RHF and CAS(10,10) methods, while the Fe–Cb and Fe–CO distances were 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 barrier calculated is equal to zero in all methods. The Fe(CO)3–Cb binding energy was calculated on CASPT2 level.