Lowest S 2 Electronic Excitations of the Boron Atom

A theoretical ab initio approach for calculating bound states of small atoms is developed and implemented. The approach is based on finite-nuclear-mass [non-Born-Oppenheimer (non-BO)] nonrelativistic variational calculations performed with all-particle explicitly correlated Gaussian functions and includes the leading relativistic and quantum electrodynamics energy corrections determined using the non-BO wave functions. The approach is applied to determine the total and transition energies for the lowest four S2 electronic excitations of the boron atom. The transition energies agree with the available experimental values within 0.2-0.3 cm-1. Previously, such accuracy was achieved for three- and four-electron systems.