COMPUTATIONAL ASPECTS OF XD. T. Richter, Institute for Crystallography, Free University of Berlin, Germany, T. Koritsanszky, S. Howard, P.R. Mallinson, Z. Su, N.K. Hansen

XD is a computer program package for multipole refinement and analysis of electron densities from diffraction data. It has been developed within an IUCr project by an iternational programing team. The main task of the package is to extract electronic properties from Bragg reflections and to perform topological analysis on 3D properties such as the electron density, its Laplacian function /1/ and the electrostatic potential. The package consists of the following programs:

*XDINI - convert data from conventional packages

*XDEDIT - edit master input file

*XDLSM - least squares program

*XDFOUR - general plane fourier program

*XDFFT - fast-fourier program

*XDPROP - electronic properties, topological analysis

*XDGEOM - geometrical analysis

*XDSTAT - statistical analysis

*XDVIB - thermal motion analysis

*XDGRAPH - graphical output

These programs will be described and their main computational aspects discussed.

/1/ R.F.W. Bader, Atoms in Molecules: A Quantum Theory, Oxford University Press (1990).