Direct methods for surface crystallography

In the most developed branches of crystallography, structure solution proceeds by two distinct steps: first, an approximate model of the structure is deduced directly from the measured data by an algorithm which assumes no preconceived model; second, a process of structure refinement simulates the experimental data for systematic variations of the parameters of such a model and determines a final structure to be that which agrees best with the data. The developments of direct methods for surface crystallography are aimed at enabling the first of these steps to be performed by an objective algorithm applied directly to the experimental data, in order to avoid the (fallible) human guesswork that has been largely applied up to the present to postulate structural models for subsequent refinement. (Some figures in this article are in colour only in the electronic version)