DIRDIF - PHASEX

PHASEX


PHASEX is a program to refine phases and expand phases of the difference structure factors (calculated for a correct set of atomic parame- ters). The difference structure factors are normalized (Wilson-Parthasarathy) giving 'E1' values, and weights are calculated (Woolfson or Sim). Reliable phases of the E1 values are input to a modified tangent formula to refine all input phases and to find phases for unphased reflections. PHASEX is particularly known for its power to solve special symmetry problems which arise from an input model which is centrosymmetric in a non-centrosymmetric space group (enantiomorph problem) or a model which has higher translation symmetry (superstructure effects). The program recognizes the problem, and uses a special symbolic addition procedure to solve the enantiomorph and/or origin ambiguity problem.

The program is used when some atoms are known (on correct positions). The program is automatically called after the execution of PATTY, ORIENT and TRACOR. However, the user should call for PHASEX when he has his own suggestions for atomic positions: for instance he may have corrected the atoms in the ATOMS file available from a foregoing run (which, of course, can only be useful if something went wrong ...).

When the user calls for PHASEX, the system will automatically arrange for structure factor calculation and normalization (program DDMAIN), then executes PHASEX followed by a Fourier synthesis (program FOUR), and finally organizes recycling several times (programs PHASEX and FOUR) for further elucidation of the structure.