Presently the following programs are in the package:
The University of Glasgow under its New Initiatives Scheme.
The International Union of Crystallography.
The package is available by subscription of US$1000 (for academic users). Information on obtaining the package is available from Tibor Koritsanszky.
TOPXD program for topological analysis of experimental static electron density based on the Hansen-Coppens multipole formalism, by Anatoliy Volkov and Carlo Gatti.
Computing School on Practical Aspects of Charge Density Determination
A beam of X-rays is diffracted by the electrons in a crystalline material, just as visible light is diffracted by larger objects. Recombination of diffracted light by means of lenses can give a magnified image of the object; X-rays, having a wavelength about four orders of magnitude shorter than that of visible light, produce an image of the electron or charge density distribution characteristic of the diffracting crystal. There exist no lenses as such for X-rays, but recombination of diffracted rays into an image can be brought about by suitable detection followed by computational Fourier transformation. The experiment is effectively an X-ray microscope for the disposition of electronic charge.
In practice we can bypass the Fourier transformation, because quantum mechanics enables us to construct a mathematical model of the charge density in a crystal. The parameters of such a model can be adjusted to reproduce the experimentally-measured pattern of diffracted X-rays, given prior knowledge of the arrangement of atomic nuclei in the crystal lattice. For chemical (as distinct from biological) molecules this can usually be found routinely using the methods of conventional crystal structure analysis programmed in widely available computer packages. This leads to a ``ball and stick'' model of the atoms and bonds representing the topology of the charge density at the level of its most salient features, found at the positions of the atomic nuclei. It is obtained by Fourier transformation of the diffracted X-ray pattern at relatively low resolution. Next we can proceed with a far more elaborate, so-called ``multipole'' model of the crystalline density, fitting it to a diffraction experiment carried out at high resolution, such that two points as close together as 0.4 x 10^{-10} m can be distinguished. As mentioned earlier, we need no Fourier transformation at this stage because the charge density in fine detail can be computed directly from the fitted multipole model. One major component of the XD package is the program for least squares fitting of a multipole model to the experimental data.
Once a charge distribution has been obtained experimentally, various chemical and physical properties that depend on the distribution can be derived. The chemical structure of molecules can be extracted from an analysis of the topology of the charge distribution, the features of which are summarized by the curvatures of the charge density at its critical points. Each feature, maximum, minimum or saddle has associated with it a point in space called a critical point, where the density is flat. One type of critical point has all three curvatures in 3-D space negative; it is found at the sites of atomic nuclei. Other types, with both positive and negative curvatures, are associated with bonding interactions between atoms. Because the strength and nature of the interactions are characterized by topology, the chemistry of the molecule can be recovered as a property of its charge distribution. A program for deriving molecular properties from the multipole model of the charge distribution is thus another major component of XD. Many of these properties can be displayed pictorially, using the 2-D and 3-