THE ELECTRON LOCALIZATION FUNCTION, A PROBE FOR STUDYING BONDED INTERACTIONS IN EARTH MATERIALS

In a study of the chemical bond, Savin et al. (1997) assert that the electron localization function (ELF) provides a novel representation of the bonded interactions for a wide variety of materials, ranging from metals to insulators. In this study, the structures of a number of common earth materials were geometry optimized, using first principles methods together with a generation of ELF isosurface maps for each optimized structure. The oxide anions of the SiOSi bonded interactions in quartz, coesite, low albite, danburite, dickite, talc, diopside and tremolite display teardrop-shaped isosurfaces ascribed to bond pair electron domains along the SiO bonds and banana-shaped isosurfaces in the nonbonded region ascribed to lone pair electron domains. The anions involved in BOB and SiOB bonded interactions display similar features whereas the AlO interactions involved in AlOSi bonded interactions display mushroom-shaped isosurfaces in the nonbonded regions with teardrop-shaped isosurfaces along the AlO and SiO bond vectors. The magnitude of an isosurface in the nonbonded region increases with decreasing SiOSi angle, indicating that the nucleophilic character of the bridging oxide anion increases with decreasing angle. The carbonate anion in calcite also displays teardrop isosurfaces along the CO bond vectors but a donut shaped isosurfaces in the nonbonded regions with two well developed maxima directed toward and coordinating the Ca ions. The banana-shaped nonbonded electron isosurfaces are also oriented toward and coordinate the nontetrahedral atoms in danburite and cordierite and to a less degree in low albite and pectolite. The OH bond vectors in dickite are directed toward the banana-shaped isosurfaces of the SiOSi bonded interactions of an adjacent Si₂O₅ sheet. The isosurfaces of the OH groups in dickite and talc matches those generated in a molecular dynamics simulation and an ELF mapping of the structure of water. The calculations indicate that the ELF provides an alternative representation for studying the bonded interactions in earth materials similar to that provided by a mapping of the Laplacian distribition of the electron density.