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 X2Σ+ state is a rather fragile van der Waals molecular ion. The binding energy of this X2Σ+ 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 A2Σ+ 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 X2Σ+ and A2Σ+ states. Graphical abstract Low-lying Ω states of cesium hydride cation.