Valence electron distribution in MgB2 studied by accurate diffraction   measurements and first principle calculations
  We use synchrotron x-ray and precision electron diffraction techniques to determine accurately the structure factors of selected reflections that are sensitive to the valence electron distribution in the superconductor MgB2. These values deviate significantly from calculated structure factors using the scattering factors of free atoms, but agree well with our calculated structure factors based on density functional theory. Having experimentally established the reliability of our firstprinciple calculated structure factors, we present electron density maps of the redistribution of the valence electrons that takes place when hypothetical free atoms of Mg and B in MgB2 interact to form the real crystal. 
Lijun Wu, Yimei Zhu, T. Vogt, Haibin Su, J. W. Davenport and J. Tafto   (Brookhaven National Laboratory)