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 [10]  cool [0.9]  Boltz [#]  ns [#]  mtpr [40]  mnqr [10]

TREF  np [100]  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.