Scattering length from accurate ab initio interaction potential of the NaLi molecule in the a³Σ⁺ electronic state

Ultracold polar and magnetic ²³Na⁶Li molecules in the rovibrational ground state of the lowest triplet a³Σ⁺ electronic state have been recently produced [1,2]. Here, we present the calculation of the electronic and rovibrational structure of these 14-electron molecules with spectroscopic accuracy (<0.5 cm^-1) using state-of-the-art ab initio methods of quantum chemistry [3]. We employ the hierarchy of the coupled-cluster wave functions and Gaussian basis sets extrapolated to the complete basis set limit. We show that the inclusion of higher-level excitations, core-electron correlation, relativistic, QED, and adiabatic corrections is necessary to reproduce accurately the scattering and spectroscopic properties of alkali-metal systems. We obtain the well depth, D_e=229.9(5) cm^-1, the dissociation energy, D₀=208.2(5) cm^-1, and the scattering length, a_s=-84(+25)(-41) bohr, in good agreement with recent experimental measurements. The scattering length is predicted without any adjustment to experimental data for the first time for alkali-metal-atom collisions. We predict the permanent electric dipole moment in the rovibrational ground state, d₀=0.167(1) debye. These values are obtained without any adjustment to experimental data, showing that quantum chemistry methods are capable of predicting scattering properties of many-electron systems, provided relatively weak interaction and small reduced mass of the system.