Low-Lying Electronic States of FeGen(-/0) (n = 1-3) Clusters Calculated with Multireference Second-Order Perturbation Theory.

The geometrical and electronic structures of the low-lying states of FeGen-/0 (n = 1-3) clusters are studied with density functional theory and state-of-the-art multiconfigurational CASSCF/CASPT2 and RASSCF/RASPT2 methods. For FeGe-/0 clusters, the CASSCF/CASPT2 results reveal that the relevant 3d, 4s, and 4d orbitals of Fe and 4p orbitals of Ge should be included into the active spaces to obtain reliable relative energy order of the low-lying states. For FeGe2-/0 and FeGe3-/0 clusters, because the active spaces raise to a size of 17 and 20 orbitals, the CASSCF/CASPT2 calculations become very time-consuming. Therefore, the RASSCF/RASPT2 method is utilized to overcome the limitations of the active space. The accuracy of RASSCF/RASPT2 with several active spaces is calibrated based on the CASSCF/CASPT2 results. The structural parameters, vibrational frequencies, and relative energies of the ground and low-lying excited states of FeGen-/0 (n = 1-3) are reported. The electron detachment energies of the anionic clusters are provided. The computed results are employed to interpret for the photoelectron spectrum of the FeGe3- cluster.