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.