Accurate Born-Oppenheimer calculations of the low-lying c ³∑_g⁺ and a³∑ _u⁺ excited states of helium dimer

Accurate variational Born-Oppenheimer calculations of the two lowest excited triplet states of the helium dimer at their respective equilibrium geometries are reported. The wave functions of the states are expanded in terms of explicitly correlated Gaussian functions with shifted centers. The obtained energies are the best variational estimates ever calculated for these states. One-electron densities are also presented and discussed. The results are compared with the experimental values and previous calculations.