Multiconfigurational quantum chemistry: The CASPT2 method

Abstract

This chapter presents the theory behind the CASPT2 method and its adaptation to a multi-state formalism. The chapter starts with an introduction of the theory of the CASPT2 method - an application of Rayleigh-Schrödinger perturbation theory applied to multiconfigurational reference function - as it was originally presented. In particular, we discuss the nature of the reference Hamiltonian and the first-order interacting space. This is followed by some detailed discussion with respect to the intruder state problem and various shift techniques to address this problem. Afterwards a longer review on alternative reference Hamiltonians, which to some degree or completely remove the intruder state problem, is put forward. Subsequently the presently proposed multi-state versions of the CASPT2 method are discussed in some detail. The chapter is concluded with a review of different benchmark assessments of the accuracy of the method and a qualified suggestion on the future development potentials of the approach.

This is a post-peer-review, pre-copyedit version of a chapter accepted and published in the book "Theoretical and Computational Photochemistry"

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