Regular no-pair Dirac operators: Numerical study of the convergence of high-order Douglas–Kroll–Hess transformations

We consider the recently developed generalized arbitrary-order Douglas–Kroll–Hess (DKH) transformation scheme for the Dirac operator. Numerical examples are presented, which demonstrate the effect of the parametrization of the unitary transformation in the high-order DKH scheme on ground state energies and 〈r−2〉〈r−2〉 momenta. Convergence of numerical results with respect to point-like and finite-nucleus potentials as well as an extrapolation scheme are considered.