Symmetry breaking in HF wave functions of Fe(CH)(2)

HF and CAS calculations for linear geometry of Fe(CH)(2) with D-infinity h symmetry have been performed. The basis sets used were DZ and DZ+P with ECP on the iron atom. Two closed-shell and one quintet RHF wave functions have been found, Phi(1)(RHF), Phi(2)(RHF) and Phi(3)(RHF(Q)). All of them are s inglet and triplet unstable in the wide range of Fe-CH distances. Singlet i nstability leads to the Charge Density Wave (CDW) broken-symmetry wave func tion with two electrons on carbon p(x) or p(y) orbital in the dissociation limit. Triplet instabilities lead to two broken-symmetry HF wave functions of Axial Spin Density Wave (ASDW) type, ASDW(1) and ASDW(2). In the dissoci ation limit they give carbon atoms with two electrons on p(x) and p(y) orbi tals coupled to singlet and triplet, respectively. The stability conditions for CDW, ASDW(1) and ASDW(2) instabilities have been derived. Other HF wav e functions with spin symmetry unrestricted have been also found. CAS(8,8), CAS(10,10) and CAS(12,12) calculations for singlet, triplet and quintet st ates of Fe(CH)(2) have been carried out. In all CAS calculations the single t state has the lowest energy. The Fe-CH equilibrium distances obtained fro m closed-shell RHF wave functions are much shorter and from broken-symmetry wave functions are much longer than those obtained from CAS calculations.