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

Abstract

HF and CAS calculations for linear geometry of Fe(CH)₂ with $D_{\infty h}$ 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, $\Phi _1^{{\text{RHF}}} ,\Phi _2^{{\text{RHF}}}$ and $\Phi _3^{{\text{RHF}}\left( {\text{Q}} \right)}$. All of them are singlet and triplet unstable in the wide range of Fe–CH distances. Singlet instability leads to the Charge Density Wave (CDW) broken‐symmetry wave function 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, ASDW₁ and ASDW₂. In the dissociation limit they give carbon atoms with two electrons on $p_x$ and $p_y$ orbitals coupled to singlet and triplet, respectively. The stability conditions for CDW, ASDW₁ and ASDW₂ instabilities have been derived. Other HF wave 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 states of Fe(CH)₂ have been carried out. In all CAS calculations the singlet state has the lowest energy. The Fe–CH equilibrium distances obtained from closed‐shell RHF wave functions are much shorter and from broken‐symmetry wave functions are much longer than those obtained from CAS calculations.