Guides to convergence

Judging convergence and assessing model quality are overlapping tasks. I will discuss criteria of convergence here. In Chapter 8,1 will discuss some of the criteria further, particularly as they relate to the quality and usefulness of the final model. 

The progress of iterative real- and reciprocal-space refinement is monitored by comparing the measured structure-factor amplitudes IFobsl (which are proportional to (/obs ) /2) with amplitudes IFca(c I from the current model. In calculating the new phases at each stage, we learn what intensities our current model, if correct, would yield. As we converge to the correct structure, the measured Fs and the calculated Fs should also converge. The most widely used measure of convergence is the residual index, or R-factor (Chapter 6, Section V.E). 

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