Excited states of Nb₃N₂ and Nb₃C₂: Density functional theory, CASSCF, and MRCI studies

Abstract

Complete active space self-consistent field (CASSCF) and multireference configuration interaction (MRCI) methods are used to investigate the Nb₃N₂ and Nb₃C₂ clusters in order to determine the agreement between multireference methods, density functional theory (DFT), and experiment. These two clusters are ideal candidates to study as the known spectroscopy can serve to validate the computational results, yet there is still room for the calculations to inform further spectroscopic experiments. We find that the MRCI leading configuration for each of the ground states is in agreement with that predicted by DFT but only accounts for up to 70% of the total configuration. CASSCF and DFT geometries are also in general agreement. Transition energies between the neutral and cationic manifolds are found to be poorly predicted by MRCI relative to the computationally cheap DFT method. For Nb₃C₂ we find that a higher energy isomer may have an electronic transition in the spectral vicinity as the lowest energy isomer.