Potential energy curve for the $a^3\Sigma_u^+$ state of lithium dimer with Slater-type orbitals

TL;DR

This paper presents highly accurate ab initio calculations of the lithium dimer's potential energy curve, including relativistic and adiabatic corrections, achieving spectroscopic accuracy without experimental fitting.

Contribution

It introduces a comprehensive ab initio approach using Slater-type orbitals and advanced correlation methods to determine the lithium dimer's potential energy with unprecedented precision.

Findings

Calculated molecular parameters with 0.2-0.4 cm$^{-1}$ accuracy

Provided ab initio scattering length for lithium atoms

Included relativistic and adiabatic corrections in potential energy

Abstract

We report state-of-the-art ab initio calculations of the potential energy curve for the $a^3\Sigma_u^+$ state of the lithium dimer conducted to achieve spectroscopic accuracy ($<$1cm$^{-1}$) without any prior adjustment to fit the corresponding experimental data. The nonrelativistic clamped-nuclei component of the interaction energy is calculated with a composite method involving six-electron coupled cluster and full configuration interaction theories combined with basis sets of Slater-type orbitals ranging in quality from double- to sextuple-zeta. To go beyond the nonrelativistic Born-Oppenheimer picture we include both the leading-order relativistic and adiabatic corrections, and find both of these effects to be non-negligible within the present accuracy standards. The potential energy curve developed by us allowed to calculate molecular parameters ($D_e$, $D_0$, $\omega_e$ etc.) for this system, as well as the corresponding vibrational energy levels, with an error of only a few tenths of a wavenumber ($0.2-0.4\,$cm$^{-1}$). We also report an ab initio value for the scattering length of two $^2S$ lithium atoms which determines the stability of the related Bose-Einstein condensate.