Lowest excitation energy of 9Be

Variational calculations employing explicitly correlated Gaussian functions and explicitly including the nuclear motion [i.e., without assuming the Born-Oppenheimer (BO) approximation] have been performed to determine the lowest singlet transition energy in the Be-9 atom. The non-BO wave functions were used to calculate the alpha(2) relativistic corrections (alpha = 1/137.035 999 679). With those corrections and with the alpha(3) and alpha(4) QED corrections determined previously by others, we obtained 54 677.35 cm(-1) for the 3(1)S -> 2(1)S transition energy. This result falls within the error bracket for the experimental transition of 54 677.26(10) cm(-1). This is the first time an electronic transition of Be has been calculated from first principles with the experimental accuracy.