(IUCr) Contents for Acta Crystallographica Section A Vol. 57 Part 2 (March 2001)

Acta Cryst. (2001). A57, 125-139 [ doi:10.1107/S0108767300013696 ]

The pair-functional method for direct solution of molecular structures. I. Statistical principles

A. D. McLachlan

Synopsis: A new statistical approach to direct methods is based on a unique many-body ensemble in which specific long-range pairing forces act between the atoms. The forces are derived from the direct correlation function of the ensemble.

Online 1 March 2001

Acta Cryst. (2001). A57, 140-151 [ doi:10.1107/S0108767300013702 ]

The pair-functional method for direct solution of molecular structures. II. Small-molecule tests

A. D. McLachlan

Synopsis: A new direct method is implemented in a simple peak-picking algorithm that alternately searches for well paired clusters of atoms and generates a new map from the strong reflections. The method solves small real molecules of up to 200 atoms. It also solves ideal point-atom models on a grid, with 600 atoms ab initio and 30000 atoms when guided by a small known fragment.

Online 1 March 2001

Acta Cryst. (2001). A57, 152-162 [ doi:10.1107/S0108767300013714 ]

The pair-functional method. III. The pairing forces

A. D. McLachlan

Synopsis: The pairing force needed to guide atoms into a structure solution is estimated by many-body theory in terms of the direct correlation function. Simple approximate formulae are derived from the Gaussian distribution, valid for all space groups. The theory is extended to treat different types of atom.

Online 1 March 2001

Acta Cryst. (2001). A57, 163-175 [ doi:10.1107/S010876730001432X ]

An exponential modeling algorithm for protein structure completion by X-ray crystallography

V. L. Shneerson, D. L. Wild and D. K. Saldin

Synopsis: An exponential modeling algorithm is developed for protein structure completion by X-ray crystallography and succesfully tested on experimental data. The Bragg reflections are phased by repeated cycles of real and reciprocal phase constraints and cross validation by means of a free R factor.

Online 1 March 2001

Acta Cryst. (2001). A57, 176-182 [ doi:10.1107/S0108767300014902 ]

On the transition from the wurtzite to the NaCl type

H. Sowa

Synopsis: The transformation from the wurtzite to the NaCl type is described by means of a deformation of a heterogeneous sphere packing. The proposed mechanism corresponds to a displacive transition without breaking bonds.

Online 1 March 2001

Acta Cryst. (2001). A57, 183-191 [ doi:10.1107/S0108767300014070 ]

Electron crystallography without limits? Crystal structure of Ti₄₅Se₁₆ redetermined by electron diffraction structure analysis

T. E. Weirich

Synopsis: The most complex structure, of Ti₄₅Se₁₆, which contains 122 atoms per unit cell, has been determined from selected-area electron diffraction data using the quasi-kinematical approach. Conditions that facilitate successful application of the quasi-kinematical approximation for electron diffraction data are discussed

Online 1 March 2001

Acta Cryst. (2001). A57, 192-196 [ doi:10.1107/S0108767300015774 ]

Strength tuning of multiple waves in crystals

S. L. Morelhão and L. H. Avanci

Synopsis: The linear polarization of synchrotron radiation is explored as a tuning key for the strength of simultaneously diffracted waves in crystals. It improves the applicability of the multiple-diffraction phenomenon for materials analysis and allows the linear polarization state of an X-ray beam to be measured from a single azimuthal scan.

Online 1 March 2001

Acta Cryst. (2001). A57, 197-200 [ doi:10.1107/S0108767300016792 ]

Quantitative determination of the spatial coherence from the visibility of equal-thickness fringes

K. Tamasaku and T. Ishikawa

Synopsis: Spatial coherence is estimated quantitatively from the equal-thickness fringes by correcting the effect of the dynamical absorption.

Online 1 March 2001

Acta Cryst. (2001). A57, 201-211 [ doi:10.1107/S0108767300018572 ]

Generalized extinction in bipyramidal crystals

G. Thorkildsen, H. B. Larsen, D. Semmingsen and Ø. Bjaanes

Synopsis: Generalized extinction factors as a function of wavelength are obtained from measurements of integrated diffracted powers by utilizing a weak reflection for scaling. The results are compared with theoretical predictions for a perfect bipyramidal crystal.

Online 1 March 2001

Acta Cryst. (2001). A57, 212-216 [ doi:10.1107/S0108767300019474 ]

A note on the problem of scattering from a single atomic plane and a stack of planes. Differences between the Ewald and other diffraction theories

P. Dub and O. Litzman

Synopsis: The scattering of a scalar plane wave (neutrons) from a single atomic plane consisting of any two-dimensional lattice with a basis is studied using the Ewald dynamical theory of diffraction. Results derived are valid for general geometries of incident and exit beams and the structure factor obtained differs from that used in the kinematic or Laue dynamical diffraction theories.

Online 1 March 2001

short communications

Acta Cryst. (2001). A57, 217-218 [ doi:10.1107/S0108767300018560 ]

X-ray determination of Debye-Waller factors and Debye temperatures of h.c.p. elements Ti, Zr, Ru, Tm, Hf

M. Shankar Narayana, N. Gopi Krishna and D. B. Sirdeshmukh

Synopsis: Debye-Waller factors of five h.c.p. elements (Ti, Zr, Ru, Tm, Hf) have been determined from X-ray diffraction intensities. The energy of formation of vacancies (E_f) has been estimated from a semi-empirical relation between E_f and [theta] .

Online 1 March 2001

Acta Crystallographica Section A

Foundations of Crystallography

Volume 57, Part 2 (March 2001)

research papers

The pair-functional method for direct solution of molecular structures. I. Statistical principles

A. D. McLachlan

The pair-functional method for direct solution of molecular structures. II. Small-molecule tests

A. D. McLachlan

The pair-functional method. III. The pairing forces

A. D. McLachlan

An exponential modeling algorithm for protein structure completion by X-ray crystallography

V. L. Shneerson, D. L. Wild and D. K. Saldin

On the transition from the wurtzite to the NaCl type

H. Sowa

Electron crystallography without limits? Crystal structure of Ti₄₅Se₁₆ redetermined by electron diffraction structure analysis

T. E. Weirich

Strength tuning of multiple waves in crystals

S. L. Morelhão and L. H. Avanci

Quantitative determination of the spatial coherence from the visibility of equal-thickness fringes

K. Tamasaku and T. Ishikawa

Generalized extinction in bipyramidal crystals

G. Thorkildsen, H. B. Larsen, D. Semmingsen and Ø. Bjaanes

A note on the problem of scattering from a single atomic plane and a stack of planes. Differences between the Ewald and other diffraction theories

P. Dub and O. Litzman

short communications

X-ray determination of Debye-Waller factors and Debye temperatures of h.c.p. elements Ti, Zr, Ru, Tm, Hf

M. Shankar Narayana, N. Gopi Krishna and D. B. Sirdeshmukh


	Cover illustration: Graphical representation of density modification due to third-order terms b¹¹³ = -b²²³ (point group $\bar{4}$ ) in the Gram-Charlier series expansion of a Gaussian atomic probability density function. Courtesy of W. F. Kuhs.

Acta Crystallographica Section A

Foundations of Crystallography

Volume 57, Part 2 (March 2001)

research papers

The pair-functional method for direct solution of molecular structures. I. Statistical principles

A. D. McLachlan

The pair-functional method for direct solution of molecular structures. II. Small-molecule tests

A. D. McLachlan

The pair-functional method. III. The pairing forces

A. D. McLachlan

An exponential modeling algorithm for protein structure completion by X-ray crystallography

V. L. Shneerson, D. L. Wild and D. K. Saldin

On the transition from the wurtzite to the NaCl type

H. Sowa

Electron crystallography without limits? Crystal structure of Ti45Se16 redetermined by electron diffraction structure analysis

T. E. Weirich

Strength tuning of multiple waves in crystals

S. L. Morelhão and L. H. Avanci

Quantitative determination of the spatial coherence from the visibility of equal-thickness fringes

K. Tamasaku and T. Ishikawa

Generalized extinction in bipyramidal crystals

G. Thorkildsen, H. B. Larsen, D. Semmingsen and Ø. Bjaanes

A note on the problem of scattering from a single atomic plane and a stack of planes. Differences between the Ewald and other diffraction theories

P. Dub and O. Litzman

short communications

X-ray determination of Debye-Waller factors and Debye temperatures of h.c.p. elements Ti, Zr, Ru, Tm, Hf

M. Shankar Narayana, N. Gopi Krishna and D. B. Sirdeshmukh

Electron crystallography without limits? Crystal structure of Ti₄₅Se₁₆ redetermined by electron diffraction structure analysis