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Hi there: I found that the grid values in the map file generated by CCP4-fft generally has a mean value of ~0, and of course there will be lots of negative values. This apparently is not the real physics, since the electron density has to be positive everywhere (hope I am right). Can somebody give me any hint how to convert the fft map file which has mean value of 0, to a more physically meaningful map which has positive densities everywhere? (I thought about offsetting the whole map by the minimum negative values to make everything positive, but I doubt it is right). Best Regards, Hailiang