Ab initio computational studies were performed for CdSe nanocrystals over a wide range of sizes and topologies. Substantial relaxations and coordination of surface atoms were found to play a crucial role in determining the nanocrystal stability and optical properties. While optimally ͑threefold͒ coordinated surface atoms resulted in stable closed-shell structures with large optical gaps, suboptimal coordination gave rise to lower stability and negligible optical gaps. These computations are in qualitative agreement with recent chemical etching experiments suggesting that closed-shell nanocrystals contribute strongly to photoluminescence quantum yield while clusters with nonoptimal surface coordination do not.

A recently developed theory of atomic-scale local dielectric permittivity has been used to determine the position dependent permittivity profiles of a few nanoscale insulator surfaces and multilayers. Specifically, slabs containing single-component ͑Si, polymer, and SiO 2 ͒ and two-component ͑Si-SiO 2 and polymer-SiO 2 ͒ systems of technological importance have been studied. The present approach indicates that the local permittivity is generally enhanced at the surfaces and/or interfaces, and that it approaches the corresponding bulk values in the interior of each component. This simple method of determining the position-dependent dielectric permittivity profiles can be used to study the impact of atomic level disorder and defects on dielectric properties.

A first-principles electronic structure study is performed to determine the optical and static polarizability tensors of various phthalocyanine ͑Pc͒ derived molecules, including H 2 Pc, CuPc, and MgPc. It is found that the dominant contribution to the polarizability is electronic in origin, and that the metal atoms only marginally enhance the polarizability. An analytical electrostatic model that relates the polarizability of an ellipsoid to its permittivity is then used to estimate the permittivity tensor of these molecular systems.

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The authors present a first principles approach for investigating the dielectric properties of Cu-phthalocyanine ͑CuPc͒. The local position-dependent dielectric constant of CuPc oligomers is determined from the charge density induced by an external finite electric field. The dielectric constants of a CuPc monomer along and perpendicular to its plane are extracted from appropriately chosen periodic arrangements of CuPc oligomers. The authors obtain dielectric constant values of about 15 along the CuPc plane and about 3.4 perpendicular to the plane.