Theoretical rovibronic spectroscopy of the calcium monohydroxide radical (CaOH)

The rovibronic (rotation-vibration-electronic) spectrum of the calcium monohydroxide radical (CaOH) is of interest to studies of exoplanet atmospheres and ultracold molecules. Here, we theoretically investigate the òΠ-X̃²Σ⁺ band system of CaOH using high-level ab initio theory and variational nuclear motion calculations. New potential energy surfaces (PESs) are constructed for the X̃²Σ⁺ and Ã²Π electronic states along with Ã-X̃ transition dipole moment surfaces (DMSs). For the ground X̃²Σ⁺ state, a published high-level ab initio PES is empirically refined to all available experimental rovibrational energy levels up to J = 15.5, reproducing the observed term values with a root-mean-square error of 0.06 cm^-1. Large-scale multireference configuration interaction calculations using quintuple-zeta quality basis sets are employed to generate the Ã²Π state PESs and Ã-X̃ DMSs. Variational calculations consider both Renner-Teller and spin-orbit coupling effects, which are essential for a correct description of the spectrum of CaOH. Computed rovibronic energy levels of the Ã²Π state, line list calculations up to J = 125.5, and an analysis of Renner-Teller splittings in the ν₂ bending mode of CaOH are discussed.