Ground state properties and optical response of Li_xNa _4-x, X = 0-4: An ab initio study

The ground state properties of Li_xNa_{4 - x} (x = 0-4) were investigated using extended basis Hartree-Fock calculations. The existence of substitution isomers was found for the mixed alkali clusters and planar singlet ground states were found for all clusters, including the isomers. The optical response of the clusters was modeled via configuration interaction - singles (CIS) in the hope of helping to interpret the experimental data. CIS results were found comparable to experiment at lower excitation energies, but inadequate at higher energies due to the neglect of doubles and higher excitations. Some exploration of the excited state surfaces of Li₄ and of one isomer of Li₂Na₂ is also reported. Comparisons are made between CIS calculations, experiment, and multireference doubly excited configuration interaction calculations.