LAUSR

Geometries and energy separations of various low-lying electronic states of an iron trimer (Fe3) are investigated by coupled-cluster singles and doubles (CCSD) and coupled-cluster singles and doubles plus perturbative triples [CCSD(T)] calculations. The ground state is found to be a 13A′ state with Cs symmetry, whereas a nearly isoenergetic state, 13A1 (C2v), is degenerate to the ground state. The ground and five low-lying states with a spin multiplicity of 13 are found below 0.20 eV at the CCSD(T) level. On the other hand, the low-lying states with spin multiplicities of 9, 11, and 15 appear only above 0.20 eV. From detailed natural bond orbital analyses, Fe3 has Fe–Fe bonds composed of σ-bond orbitals only in theβ-spin part with higher s-character in low-lying states with a spin multiplicity of 13. The polarization coefficients indicate that the σFe–Fe bonds are nearly complete covalent bonds with little polarization.