Acta Crystallographica Section D

Biological Crystallography

Volume 60, Part 7 (July 2004)

research papers

Acta Cryst. (2004). D60, 1220-1228 [ doi:10.1107/S0907444904009990 ]

Simple algorithm for a maximum-likelihood SAD function

A. J. McCoy, L. C. Storoni and R. J. Read

Abstract: Recently, the multivariate complex normal distribution has been used to develop a maximum-likelihood probability function for single-wavelength anomalous diffraction phasing and refinement of heavy-atom parameters [Pannu & Read (2004), Acta Cryst. D60, 22-27]. The function accounts explicitly for the correlations between the observed and calculated Friedel mates and their errors. However, the method of derivation of the equation described by Pannu & Read (2004) leads to a complicated likelihood expression that suffers from a number of algorithmic limitations. Here, an alternative derivation of the P_SAD function is described that leads to simplified algorithmic requirements and that allows an intuitive understanding of the expression.

Keywords: single-wavelength anomalous diffraction; multivariate complex normal; maximum likelihood; experimental phasing.