1D states of the beryllium atom: Quantum mechanical nonrelativistic calculations employing explicitly correlated Gaussian functions

Very accurate finite-nuclear-mass variational nonrelativistic calculations are performed for the lowest five D-1 states (1s(2) 2p(2), 1s(2) 2s(1) 3d(1), 1s(2) 2s(1) 4d(1), 1s(2) 2s(1) 5d(1), and 1s(2) 2s(1) 6d(1)) of the beryllium atom (Be-9). The wave functions of the states are expanded in terms of all-electron explicitly correlated Gaussian functions. The exponential parameters of the Gaussians are optimized using the variational method with the aid of the analytical energy gradient determined with respect to those parameters. The calculations exemplify the level of accuracy that is now possible with Gaussians in describing bound states of a four-electron system where some of the electrons are excited into higher angular states.