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

Abstract

Accurate description of electronic excited states of high-spin molecular species is a yet unsolved problem in modern electronic structure theory. A composite computational scheme developed in the present work contributes to solving this task for a challenging case of lanthanide-containing molecules. In the scheme, the highest-spin states whose wavefunctions are dominated by a single Slater determinant are described at the single-reference (SR) CCSD(T) level, whereas the lower-spin states, being inherently multiconfigurational in their nature, are treated with multireference (MR) methods, MRCI and/or CASPT2. An original technique which scales MR results against SR CCSD(T) ones to improve the accuracy in the former is proposed and examined, taking the example of 12 electronic states of gadolinium monoxide, X9Σ−, Y7Σ−, A′9Δ, A1′7Δ, A9Π, A17Π, B9Σ−, B17Σ−, C9Π, C17Π, D9Σ−, and D17Σ−, up to 35 000 cm−1. A multitude of the corresponding Ω (spin-coupled) states was then studied within the state-interacting approach employing the full Breit–Pauli spin–orbit coupling operator with CASSCF-generated ΛS states as a basis. For all ΛS and Ω states, the Gd–O bond lengths, spectroscopic constants ωe, ωexe, αe, and adiabatic excitation energies are obtained. The theoretical predictions are in good agreement with the experimental data, with deviations in excitation energies not exceeding 350 cm−1 (1 kcal/mol). The spectroscopic properties of the yet unobserved electronic states, A′9Δ, A1′7Δ, C9Π, C17Π, D9Σ−, and D17Σ−, are evaluated for the first time.