On the Electronic Structure of Li2 (X 1SIGMAg+) and its Changes with Internuclear Distance

A compact, yet accurate, and strictly virial-compliant ab initio electronic wavefunction for ground-state Li2 is exploited for a study of the molecule's electronic structure and electron density. Symmetry-breaking problems that emerge at the single-configuration level are solved in a multiconfigurational spin-coupled approach that enables simultaneous optimization of angularly correlated "resonating" configurations. Particular emphasis is placed on the accurate determination of the electron density's bifurcation points and of the quadrupole moment as a function of internuclear distance R. Tentative connections are drawn between the R dependence of the electron density's topological structure and quadrupole moment and that of the electronic wavefunction. Computation of the latter constitutes the first application to systems other than isolated atoms of the optimized basis set generalized multiconfiguration spin-coupled method, which entails use of nonorthogonal orbitals and Slater-type basis functions with variationally optimized exponential parameters.