A density functional theory study of the benzene-water complex.

The intermolecular interaction of the benzene-water complex is calculated using real-space pseudopotential density functional theory utilizing a van der Waals density functional. Our results for the intermolecular potential energy surface clearly show a stable configuration with the water molecule standing above or below the benzene with one or both of the H atoms pointing toward the benzene plane, as predicted by previous studies. However, when the water molecule is pulled outside the perimeter of the ring, the configuration of the complex becomes unstable, with the water molecule attaching in a saddle point configuration to the rim of the benzene with its O atom adjacent to a benzene H.

Stacking interactions and the twist of DNA.

The importance of stacking interactions for the Twist and stability of DNA is investigated using the fully ab initio van der Waals density functional (vdW-DF). Our results highlight the role that binary interactions between adjacent sets of base pairs play in defining the sequence-dependent Twists observed in high-resolution experiments. Furthermore, they demonstrate that additional stability gained by the presence of thymine is due to methyl interactions with neighboring bases, thus adding to our understanding of the mechanisms that contribute to the relative stability of DNA and RNA.

Interaction energies of monosubstituted benzene dimers via nonlocal density functional theory.

We present density functional calculations for the interaction energy of monosubstituted benzene dimers. Our approach utilizes a recently developed fully nonlocal correlation energy functional, which has been applied to the pure benzene dimer and several other systems with promising results. The interaction energy as a function of monomer distance was calculated for four different substituents in a sandwich and two T-shaped configurations.

Binding energies in benzene dimers: Nonlocal density functional calculations.

The interaction energy and minimum energy structure for different geometries of the benzene dimer have been calculated using the recently developed nonlocal correlation energy functional for calculating dispersion interactions. The comparison of this straightforward and relatively quick density functional based method with recent calculations provides a promising first step to elucidate how the former, quicker method might be exploited in larger more complicated biological, organic, aromatic, and even infinite systems such as molecules physisorbed on surfaces and van der Waals crystals.

Self-healing of CdSe nanocrystals: first-principles calculations.

Ab initio calculations of the structural, electronic, and optical properties of CdSe nanoparticles are presented. The atomic structures of the clusters are relaxed both in vacuum and in the presence of surfactant ligands. In both cases, we predict significant geometrical rearrangements of the nanoparticle surface while the wurtzite core is maintained.

Structural stability and optical properties of nanomaterials with reconstructed surfaces.

We present density functional and quantum Monte Carlo calculations of the stability and optical properties of semiconductor nanomaterials with reconstructed surfaces. We predict the relative stability of silicon nanostructures with reconstructed and unreconstructed surfaces, and we show that surface step geometries unique to highly curved surfaces dramatically reduce the optical gaps and decrease excitonic lifetimes. These predictions provide an explanation of both the variations in the photoluminescence spectra of colloidally synthesized nanoparticles and observed deep gap levels in porous silicon.

Computational studies of the optical emission of silicon nanocrystals.

We have computed absorption and emission energies of silicon nanocrystals as a function of size and of surface passivants, using both density functional theory and quantum Monte Carlo calculations. We have found that the ionic rearrangements and electronic relaxations occurring upon absorption and emission are extremely sensitive to surface chemistry. In particular, nanoclusters with similar sizes and similar absorption gaps can exhibit strikingly different emission energies.

Quantum Monte Carlo calculations of nanostructure optical gaps: application to silicon quantum dots.

Quantum Monte Carlo (QMC) calculations of the optical gaps of silicon quantum dots ranging in size from 0 to 1.5 nm are presented. These QMC results are used to examine the accuracy of density functional (DFT) and empirical pseudopotential based calculations.

Surface chemistry of silicon nanoclusters.

We employ density functional and quantum Monte Carlo calculations to show that significant changes occur in the gap of fully hydrogenated nanoclusters when the surface contains passivants other than hydrogen, in particular atomic oxygen. In the case of oxygen, the gap reduction computed as a function of the nanocluster size provides a consistent interpretation of several recent experiments. Furthermore, we predict that other double bonded groups also significantly affect the optical gap, while single bonded groups have a minimal influence.