The Douglas–Kroll–Hess electron density at an atomic nucleus

The picture-change error on Douglas–Kroll–Hess (DKH) electron densities at atomic nuclei is analyzed in detail to high orders. Our results demonstrate that high-order DKH transformations of especially the property operator are needed to obtain converged results. However, if the convergence properties are discussed in terms of a relative error a fast convergence of the picture-change corrected DKH electron density at nuclei with the DKH order of orbitals and property operator is found. Furthermore, our results suggest that already the standard low-order DKH(2,2) model is likely to yield sufficiently accurate difference densities up to the third row of the periodic table.