Ab initio MCDHF calculations of electron-nucleus interactions

We present recent advances in the development of atomic ab initio multiconfiguration Dirac–Hartree–Fock theory, implemented in the GRASP relativistic atomic structure code. For neutralatoms, the deviations of properties calculated within the Dirac–Hartree–Fock (DHF) method(based on independent particle model of an atomic cloud) are usually dominated by electroncorrelation effects, i.e. the non-central interactions of individual electrons. We present the recentadvances in accurate calculations of electron correlation effects in small, medium, and heavyneutral atoms. We describe methods of systematic development of multiconfigurationexpansions leading to systematic, controlled improvement of the accuracy of the ab initiocalculations. These methods originate from the concept of the complete active space (CAS)model within the DHF theory, which, at least in principle, permits fully relativistic calculationswith full account of electron correlation effects. The calculations within the CAS model oncurrently available computer systems are feasible only for very light systems. For heavier atomsor ions with more than a few electrons, restrictions have to be imposed on the multiconfigurationexpansions. We present methods and tools, which are designed to extend the numericalcalculations in a controlled manner, where multiconfiguration expansions account for all leadingelectron correlation effects. We show examples of applications of the GRASP code tocalculations of hyperfine structure constants, but the code may be used for calculations ofarbitrary bound-state atomic properties. In recent years it has been applied to calculations ofatomic and ionic spectra (transition energies and rates), to determinations of nuclearelectromagnetic moments, as well as to calculations related to interactions of bound electronswith nuclear electromagnetic moments leading to violations of discrete symmetries