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Electron Density Distributions and Atomic Charges

Abstract Accurate electron densities and X-ray form factors of Li, Be, F and their ions have been calculated. Electron correlation, crystal fields and ionic charge transfer change the form factors by up to a few percent, mainly in the range of sin θ/ λ < 1/3 Â -1 . Although electron correlation and crystal fields are small perturbations, their effects on the density and form factor are not additive. Densities or form factors of atomic and ionic systems are very similar; [Li0F0] and [Li+F-] procrystals differ by an effective charge transfer of not more than 0.4 e. Charge transfer and charge overlap in crystals cannot be distinguished uniquely. When the experimental data on Li2BeF4 (approximately reproduced by 3/4 atomic plus 1/4 ionic procrystal) are interpreted from the atomic partial charges are as low as 0.1 e (Li+ 0.12Be+ 0.2F- 0.14); when interpreted from the ionic viewpoint,the charges are much higher, namely 0.7 e. Intermediate viewpoints are also possible.