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IUCr 1999 Triennial Report - Commission on Charge, Spin and Momentum Densities

The Commission promotes the study of electron density distributions in both real and momentum space by bringing together physicists, chemists and crystallographers in conferences, workshops and schools, and by initiating and executing projects. The web page (http://www.tuwien.ac.at/theochem/iucr/csmd.html) is linked to the IUCr home page and contains updated information on the activities of the Commission.


1. Multipole Refinement (C. Lecomte)

During the past decades several programs have been written to carry out multipole refinements of the electron density distribution. Comparison of the results showed qualitative differences and thus made limitations apparent. This has led to the initiation of a new project for a critical assessment of the multipole refinement method. Theoretical structure factors (at T=0) were used as a benchmark to test various schemes in order to determine whether or not the different refinement methods were able to recover the original data. These tests have been tried with or without the addition of statistical errors or temperature broadening to the theoretical structure factors. The first report has been given at the Oxford Gordon Research Conference (see below) with a poster by Pillet, Souhassou, Lecomte, Schwarz, Blaha, Rerat & Lichanot.

2. The XD Program (T. Koritsanszky)

The development of the program XD by an international team under the leadership of T. Koritsanszky has been accomplished successfully and versions of this program were sent to several groups for critical tests.

3. Maximum Entropy Method (MEM) (M. Sakata)

Since 1991 the MEM has received the full attention of the community served by the Commission. Some highly controversial results have been reported at several conferences since. There was inconclusive and rather diffuse discussion about the MEM project mainly concerning the basis used in MEM. No subject led to more heated discussions than this. The need for high-quality data became apparent for successful applications of MEM. Presently it is planned to supply a few data sets, both theoretical and experimental, in order to test MEM. They will probably be for Si, Al2O3 and MgCu2 and will be made available on the web page. The first conclusion is that reliable results in precise electron density studies may be obtained if not flat prior probabilities are used.

4. Fermiology (A. Bansil)

This project focuses on the determination of the Fermiology via high-resolution synchrotron-based Compton scattering. The first step consists of standardising procedures for evaluating high-resolution Compton data. The ability of Compton scattering to contribute to the Fermiology of metallic systems is evaluated. Synchrotron-based instruments are to be combined with quantum mechanical calculations based on density functional theory. The experimental results obtained by different groups showed substantial differences, whereas on the theoretical side, quite different methodologies — FLAPW and KKR — yield highly similar results.

5. Density Matrices (W. Weyrich)

A unified quantum mechanical description of the electronic structure from experimental charge and momentum densities is attempted. The aim of the project is to investigate to what extent the combination of accurate experimental density data from both position and momentum space can enable a direct access to wavefunctions and density matrices for systems of increasing complexity. In addition to unifying position and momentum space, density matrices reveal the nature and range of chemical bonding. The possibility of obtaining information on the non-diagonal elements of the density matrix from coherent Compton scattering experiments adds to the value of the field.

Meetings, workshops and schools

Since bringing scientists from different disciplines together is one of the main objectives of the Commission, meetings play a major role in its activities. Several were organised either under the close guidance of the Commission, such as the triennial Sagamore conferences, or in some other form of cooperation, such as the Gordon Conference, or in an intermediate form of interaction.

1. The First European Charge Density Meeting

This meeting was organised by C. Lecomte and held at the Abbaye de Premontres (14—16 November 1996). Sessions included materials science, maximum entropy, high-resolution synchrotron data, electrostatics, modelling of charge density and transferability, extension of experimental work to large systems and theoretical calculations. This meeting started a new series to be held every three years.

2. XD User Group Meeting

This 'Computing School on Practical Aspects of Charge Density Determination' was held 15—18 September 1997 at FU-Berlin, Germany, and was organised by T. Koritsanszky and P. Luger. The meeting covered not only important aspects of experimental charge density determination but also theoretical considerations. It included practical aspects of data collection at low temperatures, data reduction and model refinements with emphasis on the interpretation of the results and detection of ambiguities in the procedure. Hands-on tutorials based on the XD program package were given.

3. Sagamore XII Conference

The Sagamore Conference was held in Waskesiu (in Prince Albert Park, near Saskatoon, Saskatchewan, Canada), 27 July — 1 August 1997, and was organised by B. Robertson on behalf of the Commission. This meeting was more physics oriented and showed that the field of interest is still an excellent focal point for scientists of different disciplines. Progress was noted in all parts of the field: the theoreticians discussed beyond local density methods; experimentalists, coming from neutron sources and from synchrotron facilities, brought new results on magnetisation densities; and computational scientists hotly discussed maximum entropy methods. Good progress was made with the interpretation of the experimental results.

4. The Gordon Conference

The Gordon Conference on Electron Distributions and Chemical Bonding was chaired by K. Schwarz and C. Lecomte. The meeting was organised at Queen's College in Oxford, UK, 30 August — 4 September 1998. About 72 crystallographers, theoretical chemists and physicists discussed experimental determination, quantum chemical calculation, and the interpretation and the use of electron density distributions. The area detectors, the use of the maximum entropy method and the contribution of electron diffraction raised much interest and heated discussions. The field ranged from bio-molecules to inorganic materials science applications. By electing two members of the Commission as organisers of the next conference, the participants made sure that the Gordon Conference fits nicely into the activities of the Commission.

5. Future meetings

The Second European Charge Density Meeting will be held in Sitges, Barcelona, Spain, 30 September — 2 October 1999 and will be chaired by E. Espinosa. The next Sagamore Conference will be organised by L. Dobrzynski and will be held in Poland, 1—5 September 2000. The next Gordon Conference will be organised by C. Lecomte and J. Spence in summer 2001, provided approval is obtained by the Gordon authorities. A satellite meeting to ECM-19 on Crystallographic Computing for Electron Density Analysis, chaired by N.K. Hansen, will be held in Nancy, France, 24—25 August 1999 (http://www.lc3b.u-nancy.fr/ecm19/).

Commission meetings

The Commission met in Seattle during the Congress in its new composition to discuss the next Sagamore Conference. Other meetings were held at the Sagamore Conference in Canada, where it was decided to have the next Sagamore meeting in Poland. The fields of Multipole Refinements and Maximum Entropy, Magnetisation Densities, and Encounter of Theory and Experiment in Charge Density Studies were identified for Microsymposia at the Glasgow Congress (all honoured). At the Gordon Conference a proposal was made for new candidates and the next Chair. At all meetings the projects were discussed.

A Special Interest Group on charge spin and momentum densities of the European Crystallographic Association was approved at ECM-18, Prague, Czech Republic, thanks to D. Feil and other promoters. The Chair is P. Becker.

K. Schwarz, Chair

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Updated 6th June 1999

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