High accuracy many-body calculational approaches for excitations in molecules

Two state-of-the-art computational approaches: quantum Monte Carlo and GW w ith exciton effects [GW-BSE (Bethe-Salpeter equation)] are employed to calc ulate ionization potentials, electron affinities, and first excited singlet and triplet energies for the silane and methane molecules. Results are in excellent agreement between these dramatically different approaches and wit h available experiment. The optically forbidden triplet excitation in silan e is predicted to lie roughly 1 eV higher than previously reported. In the GW-BSE method, we demonstrate that inclusion of off-diagonal matrix element s in the self-energy operator is crucial for an accurate picture.