PHASING MACROMOLECULAR STRUCTURES VIA STRUCTURE-INVARIANT ALGEBRA

Owing to the breakdown of Friedel's law when anomalous scatterers are present, unique values of the three-phase structure invariants in the whole range from 0 to 2pi are determined by measured values of diffrac tion intensities alone. Two methods are described for going from presu med known values of these invariants to the values of the individual p hases. The first, dependent on a scheme for resolving the 2pi ambiguit y in the estimate omega(HK) of the triplet (phi(H) + phi(K) + phi(-H-K ), solves by least squares the resulting redundant system of linear eq uations phi(H) + phi(K) + phi(-H-K) = omega(HK). The second attempts t o minimize the weighted sum of squares of differences between the true values of the cosine and sine invariants and their estimates. The lat ter method is closely related to one based on the 'minimal principle' which determines the values of a large set of phases as the constraine d global minimum of a function of all the phases in the set. Both meth ods work in the sense that they yield values of the individual phases substantially better than the values of the initial estimates of the t riplets. However, the second method proves to be superior to the first but requires, in addition to estimates of the triplets, initial estim ates of the values of the individual phases.