THE PROBLEM OF POTENTIAL CONSTRUCTION AND PHASESHIFT CALCULATION IN X-RAY-ABSORPTION SPECTRA THEORY OF MOLECULES AND COMPLEXES CONTAINING LOW-Z ATOMS

K x-ray absorption spectra of molecules and complexes containing low-Z atoms (CO, CO2, OCS, ferrocene) are studied. Agreement of theoretical and experimental spectra on the photon energy scale is obtained if the electron scattering phaseshifts and amplitudes are calculated with the help of the spherical wave formalism and the non-local Hartree-Fock muffin-tin (MT) potential, constructed so as to provide the potential continuity on MT sphere surfaces. In the case of molecules and complexes the approach proposed enables one to determine MT zero position in relation to the absorption threshold without any fitting procedure.