Upgrading the twin variables algorithm for large structures

Phase extension in protein molecules is presented. `Super-resolution' extension and a new figure of merit based on crystallographic symmetry are discussed.

Ill-conditioned Shake-and-Bake: the trap of the false minimum

False minima occurred frequently during Shake-and-Bake applications to large P1 data sets. These minima can be recognized and avoided with proper choice of parameter-shift conditions.

Inversion of dynamical electron diffraction data including absorption

A method to obtain the crystal potential from the scattering matrix in dynamical electron diffraction, which takes absorption into account, is proposed.

Solving the Sayre equation by simulated annealing

Simulated annealing can be used to solve the Sayre equation for ab initio structure determination. Trial calculations involving as many as 126 light atoms in the asymmetric unit are presented.

Twins and allotwins of basic mica polytypes: theoretical derivation and identification in the reciprocal space

Geometrical criteria to identify the twins and allotwins of the four basic polytypes of mica from their diffraction patterns are derived and expressed by means of the `minimal rhombus' unit in reciprocal space.

New aspects in the crystal chemistry of (M₂Cu₂O₃)_m(CuO₂)_n-type (M = Ca, Sr, Y, Bi) cuprates

The phase (M₂Cu₂O₃)(Cu_1+δO_2+γ) described as the member m/n = 1/1 in the series (M₂Cu₂O₃)_m(CuO₂)_n can be considered as an individual `Cu-deficient ladder-type' structure intermediate between SrCu₂O₃ and incommensurate phases (M₂Cu₂O₃)_m(CuO₂)_n (m ≠ n).

Expansion of scalar validation criteria to three dimensions: the R tensor

A method to expand conventional scalar validation statistics to three dimansions is described.

X-ray diffraction from an atomic plane

Reflection and transmission coefficients of X-rays by a single atomic plane are obtained in the general case where the plane consists of any two-dimensional Bravais lattice and the incident and exit X-ray beams take any direction with respect to the plane. A formula obtained for the coefficients is written in simple form.

The likelihood function in fiber diffraction

A cylindrically symmetric model is proposed for the distribution of atomic coordinate errors in helical structures. Formulas for likelihood functions in fiber difraction are presented.

Search for isotypism in crystal structures by means of the graph theory

A method for the classification of crystal structures is proposed, which is based on the representation of the interatomic bond system in a crystal as a finite `reduced' graph. The computer program IsoTest realizing this method is described.

Measurement of Debye–Waller factors in Co₃Ti by qu­antitative CBED

An effective quantitative CBED method was used to obtain the Debye–Waller factors of Co and Ti atoms in Co₃Ti. The results are B_Co = 0.20–0.24 and B_Ti = 0.41–0.43 Ų at room temperature.

Report of the Executive Committee for 1998

Patterson peaks

Nonequilibrium phase transitions in lattice models

Supersymmetry in disorder and chaos