INTEGRATED DIRECT METHOD WITH ISOMORPHOUS REPLACEMENT - SECOND NEIGHBOURHOOD OF QUINTETS. D.Subhashini and D.Velmurugan, Department of Crystallography and Biophysics, University of Madras, Madras - 600 025, India

Hauptman in 1982, worked out the probabilistic theory of three phase invariant in P1 space group for structurally isomorphous pair. Ability to use the isomorphous sets of observed magnitudes by increasing the number of unknowns, facilitates the solution of phase problem for macromolecules. Initiated with the derivation of the joint probability distribution of 30 structure factors where H+K+L+M+N=0, we present the conditional probability distsribution of the five phase structure invatiants given the 30 magnitudes in the second neighbourhood of the quintets.It remains to be tested on error-free real data.