To be noted, diffraction is not the only experimental alternative that can leverage theoretical data. For example, it is possible to measure the average electronegativity of a system by probing the ionization energies of all occupied energy levels using various techniques like ultraviolet and X-ray photoelectron spectroscopies (UPS and XPS) . This can be combined with purely computational results to evaluate the electronegativity of single atoms in a molecule or in a crystalline material .