A practical study of the electron-density-map variance

An algorithm is described for the calculation of the variance of the electron density via fast Fourier transform. The main features of the ratio electron density/standard deviation are described; it is shown that such a ratio may be considered a useful criterion for estimating, on an absolute scale, the quality of an electron-density map, no matter the quality of the model and the data resolution. In some way, it is a good estimate of the confidence one should have in the reliability of the proposed structure. The ratio was tested both for observed and for difference Fourier syntheses, in order to control the usefulness of the criterion for the most popular maps in crystallography.