High-level ab initio study of LiO (X2Π;A2Σ+) and LiO+ (X3Σ−;A3Π): the ionization energy of LiO

RCCSD(T) calculations are performed using basis sets of quadruple- and quintuple-ζ quality. Potential energy curves are calculated for the lowest two neutral states of LiO (X2Π and A2Σ+), as well as the lowest two triplet states of LiO+ (X3Σ− and A3Π). From these curves spectroscopic parameters are calculated, including anharmonic vibrational frequencies and rotational constants. For the X2Π state, excellent agreement with microwave and infrared values is obtained. The 2Σ+–2Π separation is calculated to be 2419 cm−1, and the first adiabatic ionization energy (AIE) corresponding to the process X3Σ−←X2Π is calculated to be 8.55±0.05 eV.