Ab initio calculations of the electronic structure of the doublet and quartet states of the rubidium trimer

Systematic quantum chemical calculations were performed for the ground and a number of low-lying electronically excited doublet and quartet states of the rubidium trimer molecule. The obtained potential energy surfaces (PES), spin-orbit couplings (SOC) and electronic transition dipole moments (ETDM) can be useful for optimizing paths for laser synthesis, cooling and manipulation of stable ensembles of Rb$_3$ molecules at ultralow temperatures. Ab initio calculations of the electronic structure of the homonuclear Rb$_3$ molecule, in linear, isosceles triangle and equilateral triangle geometries, were performed using the multi-reference configuration interaction method, taking into account single and double excitations (MR-CISD) and with explicit dynamic correlation of only the three valence electrons. The structure of each atom was approximated using a nine-electron effective core potential (ECP28MDF), and molecular orbitals (MOs) were optimized using the spin averaged (over doublet and quartet states) multi-configuration self-consistent field (SA-CASSCF) method. Core-valence correlations between twenty-four subvalence electrons located on doubly occupied MOs and three valence electrons were implicitly taken into account using a one-electron angular momentum-independent Muller–Mayer core polarization potential (CPP). As a result of topological investigations at over 35,000 points, two dimensional PES, SOC, and ETDM functions were obtained and the geometric parameters Rb$_3$ were found at which the most intense vertical transitions and the maximum influence of the SOC are expected.