MS-CASPT2 study of the ground and low lying states of CsH

Correlated ab initio methods [CASPT2 and R-CCSD(T)] in conjunction with the ANO-RCC basis sets in large contraction were used to calculate potential energy curves (PECs) of the ground and excited electronic states of CsH + (doublets and quartets) with the inclusion of the scalar relativistic effects and spin-orbit interaction. The ground X 2 Σ + state is a rather fragile van der Waals molecular ion. The binding energy of this X 2 Σ + state provided by both computational methods is estimated to be 0.02–0.04 eV, and is compared with the reported experimental binding energy (0.51–0.77 eV). This large binding energy can be attributed to the A 2 Σ + state, and can thus explain the apparent disagreement between theory and experiment. The spectroscopic constants of all bound states were calculated from the PECs and compared with previous published data for X 2 Σ + and A 2 Σ + states. Graphical abstract Low-lying Ω states of cesium hydride cation