A step-by-step algorithm for phase refinement with the aid of Sayre's equation

Sayre's equation is modified so as to include two kinds of atom, one kind of atom having known positions. Starting with one initial set of phases, the agreement between structure factors as calculated by Sayre's equation and the observed structure factors is systematically improved by rotating the phase angles of each of the individual structure factors through a discrete set of values; only one structure factor is rotated at a time and each is given the phase angle that produces the lowest value of the disagreement factor. This algorithm is shown to give the correct structure if some a priori knowledge is introduced into the initial set of phases.