Molecular electrostatic potentials for large systems

This chapter discusses several techniques used to investigate the reactivity of cluster surfaces and solid surfaces. The first approach used was the atomic net charges generated with different population analyses to find the reactive sites. This turned out to be insufficient. The second approach included the cumulative atomic dipole moments in the framework of a multipole expansion to describe the anisotropy of the atomic charge distributions in molecules. This technique was successfully applied to small silicon clusters. However, the multipole expansion showed a divergent behavior at the nuclei that affected the chemical bonding region. Therefore asymptotic density model (ADM) was developed to overcome the divergence problem of the multipole expansion. The use of the ADM made a fast calculation of the approximate molecular electrostatic potential for ten thousands of points possible. The same was held for the calculation of the density. An isodensity surface for a molecule was also generated very fast with this tool. The ADM was implemented in a semiempirical method and the reactivity of silicon cluster surfaces for clusters with up to 87 atoms was compared. A surface model of the reconstructed Si (111)-(7×7) surface was also created for a qualitative investigation of its reactivity. The surface model was partially saturated with hydrogen atoms. For comparison an analogous model of the unreconstructed Si(111)×(7×7) was set up.