Calculation of Low-Lying Electronic Excitations of Magnesium Monofluoride: How Well Do Coupled-Cluster Methods Work?

Abstract

Magnesium monofluoride is a polar molecule amenable to laser cooling which has caused renewed interest in its spectroscopy. In this work, we consider the case of three low-lying electronic excitations, namely X2Σ+→A2Π, X2Σ+→B2Σ+, X2Σ+→C2Σ+, using well-developed quantum chemistry approaches, i.e., without reference to the spin-orbit splitting of the A2Π states. Accurate experimental data for these transitions have been available for over 50 years. Here, we explore the linear response method at the level of CC2 theory, as well as equation of motion methods at the level of CCSD and CC3, using two families of basis sets. Excellent agreement is obtained for the first three transitions when using the correlation-consistent basis sets and extrapolation to the complete basis limit within EOM-CC3 (at a relative precision of 10−4), and qualitative agreement for the other two methods. The purpose of this paper is to serve as a guide on how to approach the accurate calculation of excitations in polar diatomic molecules.