Two-Component Relativistic Theories

Abstract

This review describes a variety of two-component methods of relativistic quantum chemistry. It starts with the description of elimination of the small component and the Pauli expansion and followed by the regular approximation method. Then the two-component methods of relativistic quantum chemistry based on the unitary transformation of the Dirac Hamiltonian are reviewed. After discussing the Douglas–Kroll–Hess method and its generalizations the main emphasis is put on the infinite-order two-component method (IOTC). The transition from the Dirac formalism to any two-component approximation is accompanied by the change of all operators, including those which correspond to external perturbations and lead to properties of different orders. This so-called change of picture problem is given some attention as well. The back-transformation of the two-component wave function to the four-component spinor and the quality of the resulting four-component wave function is discussed. Finally some discussion of the two-electron terms is presented.