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. (C) 2002 Elsevier Science B.V. All rights reserved.