IUCr 1994 Report - Commission on Charge, Spin and Momentum Densities

The Commission continued to promote the study of electron-density distributions in both real and momentum space by bringing together physicists, chemists and crystallographers, by coordinating projects and by organizing Conferences, Workshops and Schools.

Meetings of the Commission

The Commission met twice during the Sagamore XII Conference held in Brest, 7-12 August, and also held an Open Commission Meeting at which the Commission's projects were briefly reviewed and ideas for new projects canvassed. As a result of the latter activity, the Commission confirmed its wish to launch a project on the application of maximum-entropy methods to its field of interest. This project will have the active participation of M. Sakata, G. Bricogne and others. The next full Meeting of the Commission will take place during the General Assembly and Congress in Seattle, 8-17 August 1996, but a significant number of Commission members will also meet at the Gordon Conference on Electron Distributions and the Chemical Bond to be held in Plymouth, USA, 2-7 July 1995.

Projects

1. Fermiology of High-T_c Superconductors via High-Resolution Synchrotron-Based Compton Scattering. Participants in this project were engaged in planning the Workshop to be held in Himeji, Japan, at the site of SPring-8 sometime during July 1995.

2. Quantum-Chemical Description of Electronic Structure from Experimental Charge and Momentum Densities. This density-matrix project comprises essentially four areas: theory, analysis, reconstruction and direct measurement. Ten groups are now active in the area and, after a relatively slow start, impressive progress has been made. Major achievements since 1991 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 an exploration of the limits of reconstruction.

The first density matrices of solids (ionic and metallic) have been calculated and analysed (LiH, LiF, LiFHF, KFHF and Li). Reconstructions have been made of the density matrices of crystalline formamide and acetylene (from X-ray diffraction data), of gaseous neon and methane (from experimental Compton-profile data) and of the methane molecule from theoretical data for the position and momentum density with superimposed experimental noise. In addition, a model density matrix has been derived from directional Compton-profile data for LiH. Measurements have been made of some off-diagonal Compton profiles of Si with largely improved precision. In 1994, the project members organized a very successful Workshop which is described below.

3. Multipole Refinement and Related Topics. The group, which is receiving direct financial support from the IUCr for the development of a new multipole refinement package XD, is making good progress and a test version, including programs for the calculation of electrostatic and topological properties, has been released to participating groups. Discussions of the project as a whole were held during the Sagamore XI Conference (see below).

Meetings, Workshops and Schools.

1. A Workshop on Density Matrices was held in Brest at the Ecole Nationale Supérieure des Télécommunications de Bretagne (ENSTBF) overlooking the Rade, 5-6 August, just before the Sagamore XI Conference. There were 39 scientific participants, about half of them were from the field, the other half having less experience. 28 talks totalling almost 14 hours of presentation were given, and there were 3.5 hours of discussion in total. The structure of the Workshop was:
I Theory of Density Matrices (Introduction and Advances);
II Relation of Density Matrices to Specific Experiments;
III Measurement and Reconstruction of Density Matrices; and
IV Density Matrices as an Interpretative Tool.
The general view was that the talks were good throughout and presented with great enthusiasm and that substantial progress was reported. The vivid atmosphere led to a desire to discuss for even longer than the allotted 3.5 hours and there was an important exchange of information among the members of the project. A `newcomer' was heard to remark, `Now I understand much better what density matrices are and what they are good for!'

2. The triennial Sagamore XI Conference was also held at the ENSTBF near Brest. The Conference was admirably organized for the Commission by Geneviève Loupias of the Laboratoire de Minéralogie-Cristallographie, Université Pierre & Marie Curie, Paris, and Sohrab Rabii, visiting from the University of Pennsylvania. With a record of attendance of 180, the Conference demonstrated the continuing vitality of both the science and the scientists. Although at least six of the latter had attended Sagamore I, their effect on the average age of the attendees was adequately counterbalanced by the numbers of research students and young post-docs. The Conference was opened by Professor Hubert Curien, sometime Minister for Science in the French government, who spoke of the cooperation and financing required for large scientific instruments throughout the world and stressed the difficulty of obtaining long-term commitments to operational funding which would last beyond the life of a current administration.

Topics during the rest of the Meeting included the theory of X-ray and neutron scattering, X-ray dichroism and the terrain beyond the local density approximation; positron annihilation, the application of maximum-entropy methods to the reconstruction of charge and magnetization densities; magnetic scattering of neutrons and of synchrotron radiation; magnetic Compton scattering, molecular dynamics, pseudopotential calculations of ground-state charge densities, multipolar analysis, the potential of the ESRF for charge studies using hard X-rays and spin density waves in organic conductors.

Professor E. F. Bertaut, the most senior of the six who had attended Sagamore I, showed how electrostatic potentials at the surface of molecules and crystals could be calculated on the back of an envelope, or at least without a computer. We heard about charge densities and excitations, Compton scattering, including a contribution from high-energy neutron scattering, about extinction and thermal vibrations, about how to calculate charge densities and their total energies and about their topological properties and finally about fermiology and (e, 2e) spectroscopy.

During the Conference, the Commission held an Open Meeting for reports on two of its current projects: Quantum Chemical Description of Electronic Structure from Experimental Charge and Momentum Densities (leaders W. Weyrich and V. Smith Jr) and Fermiology of High-T_c Superconductors via High-Resolution Synchrotron-Based Compton Scattering Spectroscopy (leader: A. Bansil). Discussions also took place on the Multipole Refinement and Properties Project (leader: C. Lecompte), which included a report from the group that is receiving financial support from the IUCr for the development of a new multipole refinement package.

The Commission was very gratified by the lively and extensive discussion that accompanied almost every contribution and received many favourable comments about the breadth and interdisciplinary nature of the programme.

3. The Commission took an active role in helping to plan a School on Charge Densities to be held in Argentina (organizer: G. Punte). It had been hoped to hold this School in November 1994, but this was not possible and it took place 18-24 May 1995 at the Universidad Nacional de La Plata.

4. The programme for the Gordon Conference on Electron Density and the Chemical Bond to be held in 1996 has been arranged after full consultation with the Commission.

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