Cover illustration: An experimentally derived plot of the Fermi hole mobility function (FHMF) for urea. The FHMF can be interpreted as a potential energy surface for electron transfer. This plot was obtained from a Hartree-Fock wavefunction constrained to fit measured X-ray diffraction structure factors, using the Tonto program. See Jayatilaka & Grimwood [Acta Cryst. (2004), A60, 111-119].
In the resonant region near an absorption edge, a polarization anisotropy of the local X-ray susceptibility results in additional reflections otherwise forbidden by screw-axis and/or glide-plane extinction rules.
The algebraic approach to the phase problem for the case of X-ray scattering from an ideal crystal is extended to the case of neutron scattering, overcoming the difficulty related to the non-positivity of the scattering density.
The X-ray interbranch resonance concept is extended to crystals with a one-dimensional deformation. The new interbranch effect is predicted for a strongly distorted crystal with thickness of the order of the interbranch extinction length.
The 3D crystal structure of zeolites in the MWW framework family is determined from electron diffraction intensities by direct phasing using maximum entropy and likelihood. Although the `missing cone' left from incomplete goniometric data sampling leads to a somewhat distorted model, the true zeolite framework geometry may be recovered by imposing conservative bonding restraints.
Analytical expressions are derived for the polarization factors of three-wave reflections which are functions of the geometrical relations between the relevant crystallographic planes and the spectral and polarization characteristics of the incident radiation.