THE JOINT PROBABILITY-DISTRIBUTION OF ANY SET OF PHASES GIVEN ANY SET OF DIFFRACTION MAGNITUDES .4. THE ACTIVE USE OF PSI-ZERO TRIPLETS

In some recent papers [Giacovazzo, Burla & Cascarano (1992). Acta Cryst. A48, 901-906; Burla, Cascarano & Giacovazzo (1992). Acta Cryst. A48, 906-912; Cascarano, Giacovazzo, Moliterni & Polidori (1994). Acta Cryst. A50, 22-27], the method of the joint probability distribution of structure factors has been used to define a function that is frequently a maximum for the correct structure. Such a function was the basis for a modified tangent formula using P10 (negative and positive) triplet estimates and negative quartet estimates, which proved more efficient than the classical tangent formula of Karle & Hauptman [Acta Cryst. (1956), 9, 635-651]. The method is here combined with a recent formulation [Giacovazzo (1993). Z. Kristallogr. 206, 161-171], which suggests the supplementary active use in the phasing process of psi-zero triplets. Experimental tests prove the higher efficiency of the method and justify the default active use of psi-zero relationships in SIR92.