High-accuracy Rb$_{2}^+$ interaction potentials based on coupled cluster calculations

Abstract

This work discusses a protocol for constructing highly accurate potential energy curves (PECs) for the lowest two states of Rb2 + , i.e. X Σ 2 + g and (1) Σ 2 + u , using an additivity scheme based on coupledcluster theory. The approach exploits the findings of our previous work [J. Schnabel, L. Cheng and A. Köhn, J. Chem. Phys. 155, 124101 (2021)] to avoid the unphysical repulsive long-range barrier occurring for symmetric molecular ions when perturbative estimates of higher-order cluster operators are employed. Furthermore, care was taken to reproduce the physically correct exchange splitting of the X Σ 2 + g and (1) Σ 2 + u PECs. The accuracy of our computational approach is benchmarked for ionization energies of Rb and for spectroscopic constants as well as vibrational levels of the a Σ 3 + u triplet state of Rb2. We study high-level correlation contributions, high-level relativistic effects and inner-shell correlation contributions and find very good agreement with experimental reference values for the atomic ionization potential and the binding energy of Rb2 in the a Σ 3 + u triplet state. Our final best estimate for the binding energy of the Rb2 + X Σ 2 + g state including zero-point vibrational contributions is D0 = 6179 cm −1 with an estimated error bound of O(±30 cm −1 ). This value is smaller than the experimentally inferred lower bond of D0 ≥ 6307.5 cm −1 [Bellos et al., Phys. Rev. A 87, 012508 (2013)] and will require further investigation. For the (1) Σ 2 + u state a shallow potential with D0 = 78.4 cm −1 and an error bound of ±9 cm −1 is computed.