IUCr 1993 Triennial Report - Commission on Charge, Spin and Momentum Densities

In the triennium 1990-93, the guidelines of the Commission on Charge, Spin and Momentum Densities were to encourage all activities devoted to the accurate determination of density distributions in position and momentum space; within our scope we have also observed with special sympathy experimental and theoretical activities which have a strong influence on the physical understanding of ground-state properties as far as they are related to electron density distributions. This widespread activity is in accordance with the main aim of our Commission of bringing together crystallographers, whose principal interest is to investigate chemical bonding, with physicists and chemists, who contribute to the fundamental understanding of bonding, both from the quantum theoretical point of view and by utilizing different experimental methods that provide access to these problems.

The most important activity of the Commission in 1990 was the organization and realization of the following three Microsymposia at the Bordeaux Congress: Accuracy of Experimental Densities (Chair: J. Schneider), The Use of Charge Densities in Computer Simulation and Molecular Design (Chair: K. Hermansson), and Extinction (Chair: N. Kato), where the first was held as an Open Commission Meeting. The Microsymposia found appreciable support from the Congress attendees, so that the Commission felt justified with its choice of subjects. Additionally to the above Microsymposia, the Commission has organized a discussion session on Accurate Electron Densities devoted to a thorough discussion of the new Commission project on Quantum Mechanical Description of Electronic Structure from Experimental Charge and Momentum Densities. In a Closed Meeting at Bordeaux, the date and subjects of the Xth Sagamore Conference were fixed.

The most important activity of the Commission in 1991 was the organization and realization of the Xth Sagamore Conference, held 1-7 September in Konstanz, Germany, where the Commission was acting as Programme Committee. The local organization was performed very successfully by W. Weyrich and his group at Konstanz University. The Conference found good support. With 160 attendees, 45 lecture contributions (including tutorials) and 100 poster presentations, the Conference has touched the upper limits of an informal meeting with much open time for discussion and scientific exchange. The topics of the Conference covered electron and spin densities in position and momentum space and their implications both for solid-state physics and structural chemistry, where new experimental techniques, for instance magnetic photon scattering, as well as properties of new materials, for example the high-T_c superconductors and their Fermi surface topology investigated by positron annihilation, found special attention. Moreover, problems connected with the dynamics of ground-state properties (dynamic structure factor) were included in the programme following a recommendation of the preceding Sagamore Conference in 1988. In this context also new experimental developments such as ultrahigh energy resolution inelastic X-ray scattering and coherent inelastic X-ray scattering were demonstrated. Finally, the problem of reconstructing quantum mechanical quantities, e.g. density matrices, from experimental data, the matter of a new Commission project, were presented in oral and poster sessions. In two Closed Commission Meetings during the Sagamore Conference, further activities (Gordon Conference 1992, Beijing IUCr Congress, XIth Sagamore Conference 1994) were discussed, the new Commission project Quantum Mechanical Description of Electronic Structure from Experimental Charge and Momentum Densities was finally approved, and the nomination of new Commission members was initiated.

In 1992, members of the Commission have been active in contributing to the Gordon Conference, 20-24 July 1992, on Electron Distributions and Chemical Bonding, Co-chaired by M. B. Hall and G. T. De Titta, where these Gordon Conferences are mainly devoted to the chemical aspects of electron densities, thus being to some extent complementary to the Sagamore Conferences. During the Gordon Conference, a Closed Commission Meeting was held, where the nominations of new Commission members were fixed, two new Commission projects (High-T_c Fermiology, and Multipole Refinement) were approved and the Commission project Electron Density in Perovskite Structures was dropped, since it has not found enough support.

Current Commission projects:

1. Quantum Mechanical Description of Electronic Structure from Experimental Charge and Momentum Densities (Project leaders: W. Weyrich, V. Smith Jr). This project was initiated during a pre-conference Workshop of the IXth Sagamore Conference, 1988, and finally approved at the Xth Sagamore Conference. The aim of this 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, respectively. Moreover, it becomes feasible to measure on selected highly perfect crystalline solids nondiagonal elements of the density matrix in momentum space directly by means of coherent Compton scattering, so that methods for reconstructing wave functions from experimental data can be additionally controlled by experiment. The project found broad support among the attendees of the Xth Sagamore Conference in Konstanz, documented by a long list of interested persons. A meeting will be held every year, organized by the project leaders, to report on progress.

2. Fermiology of High-T_c Superconductors via High-Resolution Synchrotron-Based Compton Scattering Spectroscopy (Project leader: A. Bansil). This project was initiated during the Xth Sagamore Conference, 1991, in Konstanz. The project will consist of three steps, where the first is to standardize the procedures for evaluating high-resolution Compton data from synchrotron-based instruments. In the second step, the principal ability of Compton scattering to contribute to the fermiology of metallic systems should be investigated on rather simple systems by several groups, where the word fermiology means not only the Fermi surface but also the spectral properties of the electron gas near the Fermi energy more generally. In the third step, the participating groups, based on the experiences of the foregoing steps, should concentrate their effort on obtaining information both on the shape of the Fermi surface and on the discontinuity of the momentum distribution at the Fermi energy of YBa₂Cu₃O₇, thus making possible a fundamental contribution to the knowledge of the physics of high-T_c superconductors. In order to define more precisely how to proceed and to come to a first scientific exchange among the participating groups, a Workshop is planned to be held 2-4 July in Kraków, Poland, locally organized by L. Dobrzynski and chaired by W. Schülke and A. Bansil.

3.Multipole Refinement and Properties: a Program Package for Analysing Electron Densities from Diffraction Data (Project leader: P. Mallinson). The project was initiated at an ad hoc meeting during the Gordon Conference on Electron Distribution and Chemical Bonding, July 1992, and was approved in a ballot in December 1992. The purpose of the project is to prepare a unified version of MOLLY for analysing the electron charge distribution, where features of other currently used programs should be included. Moreover, programs for deriving properties and for analysing the topology of density distributions should be added. A comprehensive documentation of the package should be produced, and arrangements should be made for distribution and support on a continuing basis. The participation of the user community in designing updates should be invited.

1 March 1993                              W. SCHÜLKE, Chairman

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