Relativistic regular two-component Hamiltonians

Abstract

mIt is shown how the regularized two-component while many basis functions are required. If one wants to use a variational technique, one has to make sure that no spurious solutions appear. This can be done using so-called kinetically balanced basis sets, but, then, one needs an even larger basis for the small component than for the large component of the Dirac spinor. An attractive alternative is to transform the Dirac Hamiltonian into two-component form. We refer to the discussion by Kutzelnigg 111 for a detailed exposition of the various approaches and the difficulties that they give rise to in the form of divergent terms and singularities at the nuclei.The source of the difficulties is that the expansions that are being used implicitly or explicitly rely on an expansion in ( E -V )/2mc2, which is invalid for particles in a Coulomb potential, where there will always be a region of space (close to the

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