SPIN-ADAPTED MULTIPARTITIONING PERTURBATION-THEORY

The multireference generalization of the Moller-Plesset perturbation t heory employing multiple partitionings of the total Hamiltonian is rev ised in order to avoid spin contaminations and non-physical dependence of the results on the z projection of the total spin. The novel formu lation retains the main advantages of the original approach (computati onal simplicity, numerical stability, uncontracted treatment of the re ference-space part of wavefunctions, size consistency of second-order results for complete model spaces). The results of pilot calculations on the transitions between electronic states with different spin multi plicities in CH2, SiH2, ScH, Sc-2 and Cu-2 are reported.