  A methodology is developed to compute photoionization cross-sections beyond
the electric dipole (BED) approximation from response theory, using Gaussian
type orbitals and plane waves for the initial and final states, respectively.
The methodology is applied to compute photoionization cross-sections of atoms
and ions from the first four rows of the Periodic Table. Analysing the error
due to the plane wave description of the photoelectron, we find kinetic energy
and concomitant photon energy thresholds above which the plane wave
approximation becomes applicable. The correction introduced by going beyond the
electric dipole approximation increases with photon energy and depends on the
spatial extension of the initial state. In general, the corrections are below
10% for most elements, at a photon energy reaching up to 12 keV.
