In the initial stage of SAD phasing, the essential point is to break the intrinsic phase ambiguity. The presence of two kinds of phase information enables the discrimination of phase doublets from SAD data prior to density modification. One is from the heavy atoms (anomalous scatterers), while the other is from the direct-methods phase relationships. The former can be expressed by the Sim distribution, while the latter can be expressed by the Cochran distribution. Typically, only the Sim distribution has been used to yield initial phases for subsequent density modification. However, it has been demonstrated that using direct-methods phases based on the product of the Sim and Cochran distributions can lead to improved initial phases. In this paper, the direct-methods phasing procedure OASIS has been improved and combined with the SOLVE/RESOLVE procedure. Experimental SAD data from three known proteins with expected Bijvoet ratios / in the range 1.4-7.0% were used as test cases. In all cases, the phases obtained using the program RESOLVE beginning with initial phases based on experimental phases plus Sim and direct-methods information were more accurate than those based on experimental plus Sim phase information alone.

FOX [1] is a program for ab initio structure solution from powder data using global optimization algorithms working in direct space. It is a modular program, capable of using several criteria for evaluating each trial configuration. The program is optimized for inorganic materials, with the possibility of describing building blocks in the sample, and with an automatic, adaptive handling of special positions and sharing of identical atoms between neighboring building blocks (dynamical occupancy correction). It has also been tested for organic materials with a few specific optimizations to increase model convergence using improved moves. The building blocks (polyhedrons or molecules) are described with their internal coordinates (Z-matrices), thus allowing natural constraints on interatomic distances and angles. Several algorithms (Simulated Annealing, Parallel Tempering) Powder diffraction data of the high-Tc superconductor Bi-2212 were simulated based on the experimental single-crystal X-ray diffraction data by merging together unique reflections with diffraction angles (theta) closer to each other than 0.02 o . The data set contains 1879 reflections, most of them are overlapped. There are three types of overlapping, i.e. (i) overlapping of main and main reflections; (ii) overlapping of satellite and satellite reflections and (iii) overlapping of main and satellite reflections. Many overlapped reflections are mixers of all three types of overlapping. There are 89 of the total 543 main reflections without overlapping with satellites and 295 of the total 1336 satellites without overlapping with main reflections. The third type of overlapping was first separated according to the ratio between the average intensity of the 89 main reflections and that of the 295 satellites. Then the first two kinds of overlapping were treated by uniformly partitioning. Heavy-atom sites in the basic/average structure were found using the decomposed main reflections by the direct-method program SAPI. With phases calculated from the heavy atoms and structure-factor magnitudes of the decomposed main and satellite reflections, phases of the satellites were derived by the multidimensional direct-method program DIMS. Then the program VEC was used to calculate 2-dimensional sections of the 4D-Fourier map, which revealed modulation of all metal atoms as well as the saw-tooth modulation of the oxygen atom on bismuth layer. No assumed model of either the basic structure or the modulation is needed in the process. [Altomare et al. (1999). J. Appl. Cryst. 32,[339][340], is a package aimed at solving ab-initio and at refining crystal structures from a unindexed powder diffraction pattern. The new implemented procedures have greatly enhanced the originary power of EXPO. Among them we quote: 1) new techniques for indexing, via the package N-TREOR [Altomare et al., (2000). J. Appl. Cryst. 33, 1180Cryst. 33, -1186; 2) a random partition of the integrated intensities in the full pattern decomposition process [Altomare et al., (2001). J. Appl. Crys...