Quasi-dynamical electron diffraction – a kinematic type of expression for the dynamical diffracted-beam amplitudes

A kinematic type of expression is derived for the dynamical diffracted-beam amplitudes and its validity is illustrated for thin films of GaAs and Au.

Debye–Waller factors of compounds with the caesium chloride structure

Temperature-dependent Debye–Waller B factors have been obtained for five compounds with the caesium chloride structure.

A hybrid minimal principle for the crystallographic phase problem

The phase problem is formulated as the minimization of a cost function with respect to the atomic coordinates. The cost function involves both the magnitude and phase of the calculated structure factors. Structures undergoing trial calculations include hexadeca­isoleucinomycin.

Low-resolution direct phase determination in protein electron crystallography – breaking globular constraints

False low-resolution crystallographic phases for a macromolecule generated under globular density constraints can be corrected via error-correcting codes.

A model for diffraction from MCM-41 materials

A model involving only a small number of parameters provides a convenient way of interpreting diffraction patterns from MCM-41 materials. Each parameter of the model has a clear physical meaning, and this approach is clearly superior to extracting pore structure information by fitting Gaussians to an observed diffraction pattern.

The statistical kinematical theory of X-ray diffraction as applied to reciprocal-space mapping

The statistical kinematical X-ray diffraction theory is developed to describe reciprocal-space maps from deformed crystals with structural defects.

The crystallite size–disorder relationship based on the spiral paracrystal

A simple model of the crystallite size–disorder relationship for paracrystalline materials (the rule) based on the spiral paracrystal is described. Simulation results based on this model are presented.

Resolution of the phase-ambiguity problem in the centrosymmetric space group by Monte Carlo methods

Simulated-annealing Monte Carlo methods in the multistart mode were successful in overcoming the phase-ambiguity problem in space group ; for the small protein rubredoxin. This involved minimization of an energy function derived from the Sayre equation and monitoring the Terwilliger criterion characterizing the roughness of the resulting maps.

General formalism for phase combination and phase refinement: a statistical thermodynamics approach in reciprocal space

Mean-field optimization has been used to recast in a single formalism the problem of phase optimization using an arbitrary energy function in the presence of an experimentally determined phase probability function; this leads to the definition of a generalized figure of merit. Thermodynamic considerations lead to a new discussion of how to determine the relative weights in phase refinement and suggest that the free energy might be a better refinement target than maximum likelihood.

Topological analyses of cuprite, Cu₂O, using high-energy synchrotron-radiation data

Topological analyses and space partitioning were performed on high-energy synchrotron-radiation data from Cu₂O.

The experimental charge-density approach in the evaluation of intermolecular interactions. Application of a new module of the XD programming package to several solids including a pentapeptide

A new module interfaced to the XD programming package has been used in the evaluation of intermolecular interactions and lattice energies of crystals of p-nitroaniline, L-asparagine monohydrate and the pentapeptide Boc-Gln–Iva–Hyp–Ala–Phol. Comparison with ab initio supermolecular calculations at the HF, DFT and MP2 levels and with HF and DFT periodic crystal calculations shows agreement and discrepancies, the nature of which is discussed.

Solving crystal structures from two-wavelength X-ray powder diffraction data – breaking the phase ambiguity in the noncentrosymmetric case

The direct method of breaking phase ambiguity is applied in solving noncentrosymmetric crystal structures from two-wavelength powder diffraction data.

On possible extensions of X-ray crystallography through diffraction-pattern oversampling

Sampling the diffraction pattern of a finite specimen at a spacing finer than the Nyquist spacing (the inverse of the size of the diffracting specimen) corresponds to surrounding the electron density of the specimen with a no-density region. This no-density region can be used to retrieve the phase information directly from the diffraction pattern of (a) a small perfect or imperfect crystal or (b) a repeated motif without orientational regularity or (c) an unrepeated motif, such as an amorphous glass, a single molecule or a single biological cell.

New developments of the TWIN algorithm for phase extension and refinement in disordered supramolecular structures

Specific problems in disordered supramolecular structures are solved by a new development of the TWIN algorithm, through the calculation of the `super-resolution wave function'.

Report of the Executive Committee for 1999

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