Crystals User Guide

Chapter 5: Generalised Fourier Sections

Generalised Fourier sections, that is sections not parallel to a cell face, are computed by a special program (\SLANT), rather than by interpolation into the normal map. Such maps are useful for detailed examination of special problems, such as disorder, poor resolution or large thermal motion. Features identified in the map can be converted back to crystal coordinates for inclusion in the model. In spite of special computing techniques, the generalised Fourier is about 10 times as slow as the normal Fourier, for which the Beevers-Lipson method is used.

The position, orientation and extent of the map must be specified in a consistent coodinate system. This is most conveniently found from a MOLAX calculation on atoms or points in the required plane. The required position is then the centroid in orthogonal angstrom, and the matrix is the transformation from crystals fractions w.r.t. this centroid to best plane co-ordinates in Angstrom, and the extent is defined by the number of steps and the step lengths parallel to XP and YP. There are no formatting parameters available for the instruction \SLANT.

The editor \EDIT can be used to apply the rotation and translation needed to transform points back from map to cell coordinates.