Determination of the electron localization function from electron density

Approximate determination of electron localization function (ELF) from electron density and its first and second derivatives is described. It is demonstrated that the second-order gradient expansion of the kinetic energy density yields the modified ELF, which exhibits all the features characterizing electron pairing. Calculations based on the accurate electron densities derived from X-ray diffraction data carried out for crystalline magnesium oxide, chlorine and urea: they demonstrate that the ELF reveals important peculiarities of crystal architecture.