A screened static-exchange potential for core electron excitations

We explore the use of a screened potential for static exchange calculations of core electron excitations, and to which extent this potential can relie ve the main limitation of the static exchange method, namely the non-accoun t of the residual screening in the presence of the excited electron. The sc reened potential is obtained by projecting the target state on a set of pol arized orbitals, giving origin to a fully screened potential. Applications are presented and discussed for X-ray absorption spectra of a set of molecu les of different size and type. It is found that with the little extra comp utational effort using the screened potential, the low-energy part of the s pectra is generally improved, while the error in the term values are roughl y of equal magnitude but of different sign compared to the normal static-ex change calculation. Various aspects of the screened and un-screened static- exchange approximations are analyzed in comparison with other techniques. ( C) 2001 Elsevier Science B.V. All rights reserved.