International
Tables for Crystallography Volume B Reciprocal space Edited by U. Shmueli © International Union of Crystallography 2006 
International Tables for Crystallography (2006). Vol. B. ch. 2.5, pp. 286287

Convergentbeam diffraction symmetries are those of Schrödinger's equation, i.e. of crystal potential, plus the diffracting electron. The appropriate equation is given in Section 2.5.2 [equation (2.5.2.6)] and Chapter 5.2 [equation (5.2.2.1)] in terms of the realspace wavefunction ψ. The symmetry elements of the crystal responsible for generating pattern symmetries may be conveniently classified as of two types (I and II) as follows.

A minimal summary of basic theoretical points, otherwise found in Chapter 5.2 and numerous referenced articles, is given here.
For a specific zerolayer diffraction order the incident and diffracted vectors are and . Then the threedimensional vector has the patternspace projection, . The point gives the symmetrical Bragg condition for the associated diffraction disc, and is identifiable with the angular deviation of from the vertical z axis in threedimensional space (see Fig. 2.5.3.1). also defines the symmetry centre within the twodimensional disc diagram (Fig. 2.5.3.2); namely, the intersection of the lines S and G, given by the trace of excitation error, , and the perpendicular line directed towards the reciprocalspace origin, respectively. To be definitive it is necessary to index diffracted amplitudes relating to a fixed crystal thickness and wavelength, with both crystallographic and momentum coordinates, as , to handle the continuous variation of (for a particular diffraction order), with angles of incidence as determined by , and registered in the diffraction plane as the projection of .
References
Goodman, P. (1984a). A matrix basis for CBED pattern analysis. Acta Cryst. A40, 522–526.Google Scholar