Improving upon the ZORA Hamiltonian

Abstract

The metric perturbation method is used to derive what is called the normalized zeroth-order regular approximation (ZORA) Hamiltonian. This Hamiltonian, although derived in a different way, turns out to be equivalent to the infinite-order regular approximation (IORA) operator of Dyall and van Lenthe. The normalized ZORA Hamiltonian is analyzed in terms of its expansion with respect to the leading order of the fine structure constant. Through the leading second-order in the fine structure constant, the normalized ZORA Hamiltonian recovers all terms of what is known as the first-order regular approximation (FORA). The relation of the regular approximation to methods based on the Douglas-Kroll transformation is discussed. © 2006 Wiley Periodicals, Inc.