Atomic fragments defined by discrete boundaries

is slightly larger than the number of electrons in the complex. 

The violation of electroneutrality can be avoided by using a space-filling model. Such a model must fulfill the condition 

2 Radial charge density 4rcr2p(r) as a function of the radius of the sphere, based on powder data on NH4C1 (a) NH4 (b) Cl . The shaded regions represent the boundaries which lead to populations of 10.0 + 5 and 18.0 + 5 electrons for NH4 and Cl , respectively. Source Vahvaselka and Kurki-Suonio (1975). 

In the theory of metals and alloys, the Wigner-Seitz cell is defined by planes perpendicular to the interatomic vectors. Analogously, the boundary between two molecules or molecular fragments can be defined by using the relative sizes RA and RB of atom A in molecule / and the adjacent atom B in molecule II. 

Let rAB be a unit vector pointing from atom A to atom B (Fig. 6.3). To achieve the partitioning of the space between the two molecules, the vectors from atoms A and B to the point i are projected on the interatomic vector, and the ratio of the two projections is compared with the ratio of the Van der Waals radii of the two atoms. The selection criterion is 

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