IUCr 1996 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 highly successful 1995 Sagamore meeting in Brest, France, organized by G. Loupias on behalf of the Commission, showed that the field of interest is still an excellent focal point for scientists of different disciplines. The increasing accessibility of synchrotron sources has given the field a second youth by fulfilling old dreams and by enabling new experiments with unexpected accuracy. At the same time, the increase of computing facilities has continued without any sign of slowing down, with the corollary that good progress was made with the interpretation of the experimental results.

Projects

(1) Fermiology via High Resolution Synchrotron-Based Compton Scattering (A. Bansil). The project was initiated during the 1991 Sagamore conference in Konstanz, Germany. It envisages three steps, of which the first consists of standardizing procedures for evaluating high-resolution Compton data from synchrotron-based instruments with Si as test case. In the second step, the ability of Compton scattering to contribute to the fermiology of metallic systems is evaluated, while in the third step the participating groups are expected to concentrate on the momentum distribution at the Fermi energy of the high-T_c superconductor YBa₂Cu₃O₇. In the second meeting of the group, Tokyo, Japan, 1995 (see below), the reports concentrated on the first two steps. 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.

(2) Quantum Mechanical Description of Electronic Structure from Experimental Charge and Momentum Densities (W. Weyrich and V. Smith Jr). After an incubation period, the project was established during the 1991 Sagamore conference in Konstanz. 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 wave functions and density matrices. The possibility of obtaining information on the non-diagonal elements of the density matrix from coherent Compton scattering adds to the value of the field. Ten groups are active in the field. Their activities comprise four areas: theory, analysis, reconstruction and direct measurement. The major achievements have been the identification of the elements of matrices which are fixed by observables, the extension of the analysis to the Wigner and Husimi functions and to the pure momentum-density matrix and the exploration of the limits of reconstruction. Most of this work was reported at the Brest 1995 Workshop, preceding the conference (see below).

The following projects are in a final state of preparation and will start soon.

(3) Multipole Refinement. In the course of recent decades, several programs were written to carry out multipole refinements of the electron-density distribution. Comparison of the results shows differences in outcome, while at the same time a number of limitations became apparent. This led to a number of activities: (a) The development of a new program, XD, by an international team under the leadership of T. Koritszanski. The task has been accomplished successfully and versions of the new program have been sent to several groups for critical tests. (b) The initiation of a new project for critical assessment of the multipole refinement method (Claude Lecomte).

(4) Maximum Entropy Method (MEM) (Matt Sakata). Since the Konstanz Sagamore meeting in 1991, the MEM has received the full attention of the community served by the Commission. Some highly interesting results have been reported at several conferences since. No subject led to more heated discussions than this and recently it was shown that different programs lead to different results. Several questions stand insufficiently answered such as: what is the basis of the method, which algorithms should be used, how different are the results? The project is in the final state of preparation.

Meetings, Workshops and Schools

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

(1) The First International Workshop on High-Resolution Compton Scattering as a Probe of Fermiology. The meeting was organized in Krakow, Poland, 3-5 July 1994, by L. Dobrzynski, and chaired by W. Schülke and A. Bansil. It brought together theoreticians and experimentalists familiar with Compton scattering and/or synchrotron techniques. Joint approaches within the context of the project were discussed.

(2) Workshop on Density Matrices. The meeting was organized in Brest, France, 5-6 August 1994, by W. Weyrich and V. Smith Jr. It dealt with theory of density matrices, relation to specific experiments, measurement and reconstruction, and density matrices as an interpretative tool. There were 39 participants, half of which were from the field. During the conference, density matrices grew from a well defined but vaguely experienced concept to a lively tool.

(3) Sagamore XI Conference. The conference was organized in Brest, France, 7-12 August 1994, by G. Loupias. With a record attendance of 180, it demonstrated the vitality of both the science and the scientists. 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 magnetization densities, and computational scientists hotly discussed maximum-entropy methods.

(4) International School on Charge Density Analysis. The school was organized in La Plata, Argentina, 18-25 May 1995, by G. Punte. The main topics of the school were: the physical principles of accurate structure analysis, X-ray and synchrotron data-collection techniques (including advanced methods), crystallization techniques, electron-density modelling with diffraction data, error analysis of experimental results, theoretical methods in charge-density studies, chemical interpretation of the electron density in terms of orbital concepts and topological analysis, calculation properties from the multipole model, comparison of an experimental electron density with an ab initio one, application to structural activity problems. The school was attended by (mainly young) participants from Argentina, USA, UK, France, Germany, Mexico, Chile, Cuba and Uruguay. Much attention was paid to the new program package XD (Koritsansky's Project of our Commission), where much progress was noted since the Brest meeting.

(5) The Gordon Conference on Electron Distributions and Chemical Bonding. The meeting was organized in Plymouth, New Hampshire, USA, 2-7 July 1995. About 75 crystallographersand theoretical chemists discussed the experimental determination, the quantum-chemical calculation, the interpretation and the use of electron-density distributions. The use of the maximum-entropy method and the contribution of electron diffraction raised much interest and heated discussions. By electing two members of the Commission as organizers of the next conference, the participants made sure that the conference fits in well with the activities of the Commission.

(6) Second International Workshop on Compton Scattering and Fermiology. The meeting was organized by N. Shiotani in Tokyo, Japan, 28-31 August 1995. 64 scientists from 12 countries attended the three-day meeting with 44 invited talks. Synchrotron radiation has made it possible to use ultra high momentum resolution (0.02 a.u.) and photon energies up to the electron rest energy can be employed. One of the objects of the Fermiology Project is to standardize procedures with Si as test case. The results of the various groups show significant differences, probably owing to background contribution and multiple scattering. On the theoretical side, quite different methodologies - FLAPW and KKR - yield highly similar results. Compton studies on different systems were extensively discussed. Other techniques such as positron annihilation, angle-resolved photoemission and electron Compton scattering contributed to yield a robust picture of the underlying electronic structure.

Commission meetings

The Commission met in Beijing during the Congress in its new composition to discuss the next Sagamore conference. Other meetings were held at the Sagamore meeting in Brest (twice), where it was decided to hold the next Sagamore meeting in Canada because several meetings had been held in Europe in succession. The invitation to go to Poland was gratefully acknowledged and kept in mind. The fields of multipole refinements and maximum entropy, magnetization densities, and encounter of theory and experiment in charge-density studies were identified for microsymposia at the Seattle meeting (all honoured). At the Gordon conference, a proposal was made for new candidates and a Chairman. At all meetings, the projects were discussed.

10 May 1996 D. FEIL, Chairman

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