Structure solution by direct methods
Usually direct methods will be initiated with the single command TREF; for
especially easy structures one can save time with (say) TREF 20, or for large
structures brute force (e.g. TREF 2000) may prove necessary. In fact there
are a large number of parameters which can be varied, though the program is
based on experience of many thousands of structures and can usually be relied
upon to choose sensible default values. A summary of these parameters appears
after the data reduction output, and should be consulted before attempting any
direct methods options other than 'TREF n'.
The phase refinement of multiple random starting phase sets takes place in
three stages, controlled by the INIT, PHAN and TREF instructions. The 'best'
solution is then expanded further by tangent expansion and E-Fourier recycling
(see the section on partial structure expansion).
INIT nn [#] nf [#] s+ [0.8] s- [0.2] wr [0.2]
PHAN steps  cool [0.9] Boltz [#] ns [#] mtpr  mnqr 
TREF np  nE [#] kapscal [#] ntan [#] wn [#]
Only the TREF instruction is essential to specify direct methods;
appropriate INIT, PHAN, FMAP, GRID and PLAN instructions are then generated
automatically if not given.