Shapes of independent atoms and chemical deformation densities of second-row molecules

It is suggested that the information about chemical bonding contained in the total electron density distribution should be split into three parts. First, the standard crystallographic parameters (i.e. the atomic positions which are most appropriate for bond formation); these give rise to the standard "total difference density" (TDD). Second, the shapes of degenerate atomic ground states appropriate for optimal interatomic interactions, giving rise to the novel "chemical deformation density" (CDD). Third, the genuine chemical deformation of the atoms in the molecule or crystal. This approach was applied to more than 40 molecules consisting mainly of second-row atoms (lithium to fluorine) and hydrogen. Contrary to the standard TDD, the novel CDD gives a very consistent pattern of covalent bond and lone pair densities, even for electron-rich atoms. The atomic shape parameters comprise valence atomic orbital directions and populations. They correlate very well with the type of bonding of the atoms in the molecule.