The interest of describing the ground state properties of a system in
terms of one electron density (or its two spin components) is obvious,
in particular due to the simple physical significance of this function.
Recent experimental progress in diffraction made the measurement of
charge and magnetization densities in crystalline solids possible, with
an accuracy at least as good as theoretical accuracy. Theoretical
developments of the many-body problem have proved the extreme importance
of the one electron density function and presently, accurate methods of
band structure determination become available. Parallel to the
diffraction techniques, other domains of research (inelastic scattering,
resonance, molecular spectroscopy) deal with quantities directly related
to the one particle density. But the two types of studies do not
interfere enough and one should obviously gain more information by
interpreting all experiments that are related to the density together.
It became necessary to have an International School that reviews the
status of the art in the domain of "ELECTRON AND MAGNETIZATION DENSITIES
IN MOLECULES AND CRYSTALS". This was made possible through the generous
effort of N.A.T.O. 's Scientific Affairs Division, and I would specially
thank Dr. T. KESTER, the head of this Division, for his help and
competence. An Advanced Study Institute was thus held in ARLES, south
France, from the 16th to the 31st of August 1978.