Geometries and electronic states of iron trimer (Fe3) by CCSD and CCSD(T) calculations

摘要

Geometries and energy separations of various low-lying electronic states of an iron trimer (Fe 3 ) 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 13 A ′ state with C s symmetry, whereas a nearly isoenergetic state, 13 A 1 (C 2v ), 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, Fe 3 has Fe–Fe bonds composed of σ -bond orbitals only in the β -spin part with higher s-character in low-lying states with a spin multiplicity of 13. The polarization coefficients indicate that the σ Fe–Fe bonds are nearly complete covalent bonds with little polarization.