MXAN: a new software procedure to perform geometrical fitting of experimental XANES spectra

A new software procedure, MXAN, to fit experimental XANES spectra is presen ted here. The method is based on the comparison between the experimental sp ectrum and several theoretical calculations generated by changing the relev ant geometrical parameter of the site around the absorbing atom. The x-ray photoabsorption cross section is calculated using the general multiple-scat tering scheme, utilizing a complex Hedin-Lunqvist energy-dependent potentia l to describe the exchange correlation interaction. Our method has been app lied to the study of geometrical environment of the tetrahedral zinc site o f the protein superoxide dismutase (SOD). The experimental Zn K-edge XANES spectrum has been fitted in the space of the first shell coordination param eters (ligand distances and angles) following the behavior of the chi-squar e as a function of the local distortion from the starting crystallographic structure. The recovered structure is found to be independent on the starti ng conditions, showing the theoretical uniqueness of the structural solutio n. Strengths and limitations of the application to real systems are also di scussed.