A multireference valence bond approach to electon excited states

A new spin coupled multireference balence bond scheme for the treatment of electronic excited states is presented. The method involves the specific optimization of excited spin coupled wave functions which act as reference functions and are used to introduce correlation corrections directly in the excited state of interest. The variational optimization of such excited wave functions is achieved by using simple orthogonality constraints on the spin coupled orbitals and it preserves all the appealing features of the valence bond method. The applications to the lower lying states of [LiH2](+) and BeH show the advantages and some of the limitations of this new approach.