XD: From Multipolar Model Refinement to Complete Topological Analysis. A description of the new features

The program package XD was the result of a project by the IUCr commission on charge spin and momentum density. It was first distributed in 1995 [ ] and afterwards adopted in many laboratories devoted to X-ray charge density studies [ ]. Since then, the topological analysis of the experimental electron density has been more extensively adopted and the quantum theory of atoms in molecules [ ] has been used in many publications as the paradigm for interpreting electron density distributions. This forced to develop the program in order to allow a full topological analysis, including the mapping of atomic basins and the integration of atomic properties. For this reason, the program TOPXD [ ] was released, by combining the multipolar expanded electron density from XD and the topological analysis in the crystal performed by TOPOND [ ]. TOPXD is now fully integrated in XD [ ], as one of its main section. Other new features are now included in the program: the integration of charges and multipole moments with the Stockholder partitioning [ ] and several new options of the XDPROP section, a fast Fourier transform peak search (XDFFT), a more complete analysis of the refined model and the availability of wave functions fitted to calculations at relativistic level (and extended up to Xe) [ ].