Bonded and nonbonded electron lone pairs for a silicate molecule

Bonded and nonbonded electron lone pairs for a silicate molecule 

An examination of the 25 e/A -isosurface reveals that it is not only crescent-shaped, but that is split into a crescent dumb-bell shaped feature that extends part way around the anion. As such, it indicates that the electron density is locally concentrated in two Ip domains connected by an intervening region where the electron density is locally concentrated but to a much lesser degree. Thus, the oxide anion, when bonded to an H and Si atom, exhibits two lone pair domains rather than an elongate one as found when it is bonded to two Si atoms. 

The electron localization function (ELF) is another tool that has been used with considerable success in highlighting domains in p(r) of strong electron localization (Becke and Edgecombe 1990). These domains, like those defined by the maxima in VSCC, have likewise been associated with the bonding and nonbonding electron-pairs of the VSEPR model (Becke and Edgecombe 1990 Bader et al. 1996 Bader et al. 1996 Savin et al. 1997). The ELF has also been found to reveal the shell structure of an atom in a clear and faithful fashion. 

ELF values for the valence electron distributions for quartz, coesite and stishovite have been calculated at an isosurface ELF level of 0.844 (Gibbs et al. in prep). For each of these silica polymorphs, localized domains of electron density were found along each 

In stishovite, each oxide anion is bonded to three Si cations and lies in the plane defined by the cations. The anion was found to be coordinated by five well-defined domains of electron localization (Gibbs et al., unpublished data). Of these, three were found to be displaced slightly off the SiO bond vectors toward the exterior of the shared edges as observed in deformation maps (Spackman et al. 1987) The remaining two were 

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