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 A ̃ 2 Π – X ̃ 2 Σ + band system of CaOH using high-level ab initio theory and variational nuclear motion calculations. New potential energy surfaces (PESs) are constructed for the X ̃ 2 Σ + and A ̃ 2 Π electronic states along with A ̃ – X ̃ transition dipole moment surfaces (DMSs). For the ground X ̃ 2 Σ + 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 A ̃ 2 Π state PESs and A ̃ – X ̃ 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 A ̃ 2 Π 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. [ABSTRACT FROM AUTHOR]