Ab initio calculation of excited state energies using basis set optimization and open-shell Mّller-Plesset perturbation theory

For high-spin open-shell systems a consistent approach for determining the correlation energy of ground and excited states (ES) is proposed. An effective method of taking the orthogonality constraints into account is used to construct an optimal single reference wave function (WF) for the ES. On the one hand, it is an eigenfunction of Ŝ2 and, on the other hand, the WF is determined by means of an unrestricted Hartree-Fock scheme. This allows one to develop an open-shell perturbation theory based on the well-defined zeroth-approximation for the ES and to keep the advantages of the genuine Møller-Plesset PT. A comparison with other methods is made. The potential curves of low-lying ES for HeH are presented to test the performance of the proposed method. Basis set optimization was found to be essential to achieve correct results for the ES.