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.