Relativistic Equations of Motion Method

Abstract

A systematic approach to a relativistic theory of electronic excitations in atoms and molecules is discussed. The methodology is based on the relativistic equations of motion for excitation processes. For low-Z systems, relativistic and radiative corrections can be calculated in Breit-Pauli approximation. For medium and high-Z systems the “no-pair” Hamiltonian is chosen as the unperturbed Hamiltonian, the residual interaction is regarded as a perturbation and quantum electrodynamical effects are calculated within the framework of perturbation theory. The second-order correction to the excitation energy, due to creation and destruction of virtual electron-positron pairs, is discussed. Radiative corrections to the excitation energy are calculated as first-order corrections arising from the use of the Breit operator. Detailed expressions for the Breit interaction contributions are derived when the excitation operator is approximated by the first-order perturbation theory solution of the RPA equations.