Accuracy Douglas-Kroll-Hess calculations

The example of neon, where relativistic contributions account for as much as a0.5% of 711, shows that relativistic effects can turn out to be larger for high-order NLO properties and need to be included if aiming at high accuracy. Some efforts to implement linear and nonlinear response functions for two- and four-component methods and to account for relativity in response calculations using relativistic direct perturbation theory or the Douglas-Kroll-Hess Hamiltonian have started recently [131, 205, 206]. But presently, only few numerical investigations are available and it is unclear when it will become important to include relativistic effects for the frequency dispersion. 

Abstract In this chapter I discuss some aspects of relativistic theory, the accuracy of the infinite order two-component relativistic lOTC method and its advantage over the infinite order Douglas-Kroll-Hess (DKHn) theory, in the proper description of the molecular spectroscopic parameters and the potential energy curves. Spin-free and spin dependent atomic mean filed (AMFI) two-component theories are presented. The importance of the quanmm electrodynamics (QED) corrections and their role in the correct description of the spectroscopic properties of many-electron atoms for the X-ray spectra is discussed as well. Some examples of the molecular QED calculations will be discussed here as well. 

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