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 relieve the main limitation of the static exchange method, namely the non-account of the residual screening in the presence of the excited electron. The screened potential is obtained by projecting the target state on a set of polarized orbitals, giving origin to a fully screened potential. Applications are presented and discussed for X-ray absorption spectra of a set of molecules of different size and type. It is found that with the little extra computational effort using the screened potential, the low-energy part of the spectra is generally improved, while the error in the term values are roughly of equal magnitude but of different sign compared to the normal static-exchange 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.