  A function has been proposed to evaluate the electron density model
constructed by inverse Fourier transform using the observed structure
amplitudes and trial phase set. The strategy of this function is applying an
imaginary electron density modification to the model, and then measuring how
well the calculated structure amplitudes of the modified model matches the
expected structure amplitudes for the modified correct model. Since the correct
model is not available in advance, a method has been developed to estimate the
structure amplitudes of the modified correct model. With the estimated
structure amplitudes of the modified correct model, the evaluation function can
be calculated approximately. Limited tests on simulated diffraction data
indicate that this evaluation function may be valid at the data resolution
better than 2.5 {\AA}.
