A simple method for obtaining electron scattering phase shifts from energies of an atom in a cavity

We present a simple method for obtaining elastic scattering phase shifts and cross sections from energies of atoms or ions in cavities. This method does not require calculations of wavefunctions of continuum states, is very general, and is extremely convenient from practical point of view: some conventional computer codes designed for the energies of bound states can be used without modifications. The application of the method is illustrated on an example of electron scattering from Kr and Ar. From Brueckner orbital energies in variable cavities, we have obtained ab initio cross sections that are in close agreement with experiment. The relativistic effects are also considered and found to be small below 10 eV.