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The core theme of X-ray crystallography is reconstructing the electron density distribution of crystals under the constraints of observed diffraction data.Nevertheless,the reconstruction of electron density distribution by straightforward Fourier synthesis is usually hindered due to the well-known phase problem and finite resolution of diffraction data.In analogy with optical imaging system,the reconstructed electron density map may be regarded as the image of the real electron density distribution in crystals.Inspired by image definition evaluation functions applied in auto-focusing process,we proposed two evaluation functions for the reconstructed electron density images.One of them is based on atomicity of electron density distribution and properties of Fourier synthesis.