A route to high-accuracy ab initio description of electronic excited states in high-spin lanthanide-containing species: A case study of GdO.

This article focuses on developing a composite computational scheme that combines single-reference coupled cluster (CCSD(T)) and multireference (MRCI and CASPT2) methods to accurately describe the electronic excited states of high-spin lanthanide-containing molecules, exemplified by gadolinium monoxide (GdO). The approach treats high-spin states with CCSD(T) and lower-spin, multiconfigurational states with scaled multireference calculations, improving accuracy and computational feasibility. The study provides highly accurate spectroscopic constants, excitation energies, dipole moments, and spin–orbit coupled states for 12 ΛS and 79 Ω electronic states of GdO, showing excellent agreement with experimental data and refining assignments of several electronic states. The authors suggest that this composite protocol can be extended to other lanthanide- and transition metal-containing diatomics, offering a balanced and resource-efficient method for modeling complex electronic spectra.