Multiconfiguration method for excited states of atoms and molecules

A new method is presented by which the orthonormal orbital set of a multiconfiguration wave function can be optimized for an excited state of a given symmetry and spin, such that the calculated excited state energy remains an upper bound to the corresponding exact energy. The common difficulties of root flipping and poor convergence are avoided by isolating the desired root by a root-shifting mechanism. Model calculations on the lower 2S states of Li and the 1S states of Be demonstrate the capability and stability of this method