Pictorial Pattern Recognition and the
Phase Problem of X-ray Crystallography

The availability of interactive, three-dimensional,
computer graphics systems coupled to powerful 
digital computers encourages the development of algorithms
adapted to this environment.  Pictorial pattern 
recognition techniques make possible a number of approaches
to X-ray structure determination based on 
molecular model building, i.e. the use of chemical information
to frame "structural hypotheses" which 
can computationally be tested and refined by reference
to the experimental data.  Application of standard 
pattern recognition algorithms is hindered by the fact
that the cross-correlation between a model and 
the correct structure cannot be computed because of
a fundamental incompleteness in the measured data. 
 However, it is possible to compute an upper bound to such
a cross-correlation.  A simple example demonstrates 
that this information can be the basis of a technique
for structure determination that can make effective 
use of an interactive graphics system. Model building
by cross-correlations has intrinsic advantages 
over usual crystallographic techniques based on the
autocorrelation or Patterson function, especially 
for large structures.  This is significant, for crystallography
of biological macromolecules hasbeen 
and will continue to be a field of intense interest.

CACM January, 1972

Lesk, A. M.

pictorial pattern recognition, phase problem,
X-ray crystallography, interactive graphics

3.13 3.17 3.63

CA720101 JB February 1, 1978  9:53 AM

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