Relativistic Multireference Perturbation Theory: Complete Active-Space Second-Order Perturbation Theory (CASPT2) With The Four-Component Dirac Hamiltonian

Abstract

The relativistic complete active-space second-order perturbation theory (CASPT2) developed for the four-component relativistic Hamiltonian is introduced in this chapter. This method can describe the near-degenerated and dissociated electronic states of molecules involving heavy elements. This method is applicable for the systems which can be described by neither DFT nor single reference methods, and the system with very heavy-elements which cannot be described by quasi-relativistic approaches. The present theory provides accurate descriptions of bonding or dissociation states and of ground and excited states in a well-balanced way. In this review, for example, the ground and low-lying excited states of diatomic molecules with 6p series, TlH, Tl₂, PbH, and Pb₂ are calculated with the Dirac–Coulomb (DC) CASPT2 method and their spectroscopic constants and potential energy curves are presented. The obtained spectroscopic constants are compared with experimental findings and previous theoretical works. For all the molecules, the spectroscopic constants of DC-CASPT2 show reasonably good agreement with the experimental or previous theoretical spectroscopic constants