  Short-range corrections to long-range selected configuration interaction
calculations are derived from perturbation theory considerations and applied to
harmonium (with two to six electrons for some low-lying states). No fitting to
reference data is used, and the method is applicable to ground and excited
states. The formulas derived are rigorous when the physical interaction is
approached. In this regime, the second-order expression provides a lower bound
to the long-range full configuration interaction energy. A
long-range/short-range separation of the interaction between electrons at a
distance of the order of one atomic unit provides total energies within
chemical accuracy, and, for the systems studied, provide better results than
short-range density functional approximations.
