Relativistic Pseudopotentials: Their Development and Scope of Applications

The most important aspects of the theoretical background of effective core potentials (ECP), which include the pseudopotential (PP) and the model potential (MP) approach, are described. The commonly used ECP method does not only have the advantage to reduce the computational effort compared to an all-electron (AE) calculation by limiting the number of electrons treated explicitly but also allows an implicit incorporation of the most important relativistic effects. The bond contractions and expansions may occur parallel to orbital contractions and expansions but cannot necessarily be explained to be caused by them. The MP approach for atoms represents an important link between the full AE calculation and the PP calculations. Semiempirical PPs and corresponding CPPs were generated by Igel et al. for most of the main group elements by fitting the spectra of the one-valence electron ions for PP and CPP.