Configuration selection in the MC SCF method. The van der Waals X 2Σ + state of LiHe

Configuration selection methods for multiconfiguration (MC) SCF wavefunctions have been applied to the van der Waals X 2Σ + ground state of LiHe. The configuration selection was restricted to “dispersion” configurations, corresponding to simultaneous single excitations out of each atom. Energy lowering or CI coefficient selection led to similar results. An MC SCF wavefunctionconsisting of thirteen configurations gave for the van der Waals minimum of LiHe R e = 12.70 bohr (11.72/12.04) and a well depth ϵ = 2.98 μhartree (2.70/3.69 ), with two sets of fits to experimental data in parentheses.