Re: NCS worries

["Hazes B." <bart@mrc-lmb.cam.ac.uk>]

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Two people have pointed out to me that calculating a "self-Patterson" 
using your observed intensities and all phases set to zero" is strange
since intensities don't have phases. I agree. However, to calculate a
Patterson you have to generate map coefficients by multiplying the
structure factor with its complex conjugate which gives coefficients that
are the square of the original structure factor amplitude and a phase of
zero. Since the square of the structure factor amplitude is proportional
in magnitude to the measured intensity this means that you can use the
intensity for this calculation, but of course you can use F-squared if you
prefer (perhaps your preference depends on the FFT program you are using
to calculate the map?). 

It may be useful to calculate your "self-Patterson" at relatively low
resolution (e.g. 6 to 999 Angstrom) because you will then be able to spot
pseudo translational symmetry even if it is very pseudo.

Bart