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 PHASeless f

When this keyword is present and the calculation performed is a FOM-weighted synthesis, then all reflections with a figure of merit less than f will enter the calculation with only amplitude restraints, but no phase restraints. The idea behind it, is that due to the algorithm used by GraphEnt (discussed in page 14), reflections with a high F/$ \sigma$(F) but low FOM, will effectively be treated as weak. By removing the phase restraints, strong reflections (but with largely unknown phase) will be treated as strong reflections with unknown phase. The hope is that their phases will converge to a value that maximises the configurational entropy of the map (while being consistent with their amplitudes). Now, this whole thing (of arbitrarily selecting a FOM cutoff to switch-off phase restraints) is clearly an ad hockery which is neither promising, nor convincing (but you can give it a try just in case). This is more extensively discussed in section 16.8. I should add (just to avoid any misunderstandings), that the iteration is started using the phase angles given to the program ( ie. it is not a random-phase seeded calculation), and that the FOMs given are not ignored but are used to adjust how fast the corresponding amplitudes will approach convergence.


next up previous contents
Next:  SWITch f Up: CALCULUS AND LIMITS-RELATED KEYWORDS. Previous:  TARGet f   Contents
NMG, March 2000