Ab initio and perturbative calculations of the electric susceptibility of atomic hydrogen

We apply a numerical basis-state method to determine the electric susceptibility of atomic hydrogen via calculations of the terms of the perturbative power series expansion up to eleventh order as well as from ab initio calculations of the field-induced polarization. The results of the perturbative calculations indicate that the applicability of the power series expansion is limited to intensities of a uniform electric field below 2 x 10(13) W/cm(2) at a wavelength of 800 nm. While the predictions for the electric susceptibility from the perturbative power series tend to strongly increase beyond this intensity regime, those from ab initio calculations have a negative slope as a function of intensity. We therefore interpret that the latter feature is due to the nonperturbative interaction between the atom and the external field and an explanation via higher-order nonlinear terms is not applicable.