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 Using the programme.

To run the programme type run dsk$21:[public.leeds.nmg.exe]maxent.exe. Alternatively, type @dsk$21: [public.leeds.nmg.com]login.com and this will define the symbol maxent which you can use to run the programme.


CAUTION (1) : The exponential in the basic iteration formula (shown on the first page) makes the algorithm unstable for large values of $ \lambda$. Although the programme will scale the input data and give $ \lambda$ a sensible initial value ( $ \lambda_{initial}^{}$=0.1), it is still possible that the iteration will get out of control and the programme crash. If this happens, you must decrease the $ \lambda_{initial}^{}$ as described in the section ``Changing the defaults''. It is also possible that the MAXENT algorithm can not converge to the expected value of $ \chi^{2}_{}$ with a given $ \lambda_{initial}^{}$. In this case, you again have to decrease $ \lambda_{initial}^{}$.

CAUTION (2) : The map files must be calculated with the most recent MTZ version of the CCP4 suite.

Note well (1) : In the case of crystallographic applications, the input image must be one unit cell (not the asymmetric unit).


Note well (2) : The programme uses the Danielson & Lanczos FFT algorithm (from Numerical recipes, originally written from N.M.Brenner). This means : (i) the number of grid points in each dimension of the input map(s) must be a power of 2, and, (ii) assuming constant convergence properties (which is not true), MAXENT is a O(Nlog2N) process, where N is the number of grid points in the image. Calculating the MAXENT map of a 128x32x64 image (262144 grid points) took 2 hours and 15 minutes of CPU time on a DEC 4000. The same calculation for a 128x128 2D image took 10 minutes of CPU time on the same machine.


$\textstyle \parbox{4.5cm}{ {\bf Scenario A :}\newline {\bf Unknown sigmas.} }$ $\textstyle \parbox{11cm}{ If you have $\sigma(F)$s it is strongly recommended t...
...le to modify the value of $k$\ (see section \lq\lq Changing the defaults'').
\par
}$

$\textstyle \parbox{4.5cm}{ {\bf Scenario B :}\newline {\bf Known sigmas.} }$ $\textstyle \parbox{11cm}{ The image will be read from a {\bf IMAGE.MAP} map fil...
....com file otherwise the $F$s and $\sigma(F)$s will not be on the same scale}. }$

$\textstyle \parbox{4.5cm}{ {\bf Changing}\newline {\bf the defaults} }$ $\textstyle \parbox{11cm}{ If a {\bf MAXENT.DAT} file is present in the default ...
...nes
\par
~~~~~~~~VERBOSE
\par
~~~~~~~~LAMBDA~0.05
\par
~~~~~~~~SIGMA~~0.20
}
}$


next up previous contents
Next: About this document ... Up:  Old (obsolete) version's documentation Previous:  Results.   Contents
NMG, March 2000