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#

A word of caution

I should warn you right from the beginning
that the algorithm used in * GraphEnt* is neither the most stable nor the
most efficient of those published^{3} (but it is the one that is the easiest to code). Additionally, for those
cases where the calculation includes a known figure-of-merit for the phase angles, * GraphEnt* is
performing additional approximations which although I hope that
are generally safe, they do not represent
the best that can be achieved with the data.

I should also warn you that the amount of time that the calculation may require depends on the
input data quality
and there is no * a priori* guarantee that the given algorithm will converge
even if given enough time. Having said that, a 262,144 (=128x64x32) pixels
* GraphEnt*
*mF*_{o}exp(*i*)
map corresponding to a reasonably accurate data set could be calculated in less than
8 minutes of CPU time on a DEC Alpha 1200, a 524,288 (=128x128x32) pixels * GraphEnt* difference Patterson map for a loosy derivative (which makes the calculation easy)
took only 46 seconds on the same machine, and a 2Å
(2*mF*_{o} - *DF*_{c})exp(*i*) synthesis with
3,072,000 (=160x160x120) pixels took 40min. Finally, a 2-0.8Å
(2*F*_{o} - *F*_{c})exp(*i*) synthesis with 9,437,184 (=192x256x192) pixels
for a 4--helical bundle protein took only 13 minutes on a
Pentium 800MHz (but the data were, of course, rather weak at this resolution
range).

#### Footnotes

- ... published
^{3}
- See for example
* Maximum Entropy and
Bayesian Methods in Inverse Problems* (1985), edited by Smith, C.R. & Grandy, W.T., Jr.,
Dordrecht : Reidel.

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NMG, Nov 2002