MCSCF wave functions for excited states of polar molecules: Application to BeO

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Abstract
A previously reported multiconfiguration self-consistent field (MCSCF) algorithm based on the generalized Brillouin’s theorem is extended in order to treat excited states of polar molecules. In particular, the algorithm takes into account the proper treatment of nonorthogonality in the space of single excitations and invokes, when necessary, a constrained optimization procedure to prevent variational collapse of exited states. In addition, a configuration selection scheme (suitable for use in conjunction with extended configuration interaction methods) is proposed for the MCSCF procedure. The algorithm is used to study the low-lying singlet states of BeO, a system which has not previously been studied using an MCSCF procedure. MCSCF wave functions are obtained for three 1Σ+ and two 1Π states. The 1Σ+ results are juxtaposed with comparable results for MgO in order to assess the generality of description presented here.