Aaron Puzder

Van der Waals density functional: Self-consistent potential and the nature of the van der Waals bond

We derive the exchange-correlation potential corresponding to the nonlocal van der Waals density functional [M. Dion, H. Rydberg, E. Schroder, D. C. Langreth, and B. I. Lundqvist, Phys. Rev. Lett. 92, 246401 (2004)]. We use this potential for a self-consistent calculation of the ground state properties of a number of van der Waals complexes as well as crystalline silicon. For the latter, where little or no van der Waals interaction is expected, we find that the results are mostly determined by semilocal exchange and correlation as in standard generalized gradient approximations (GGA), with the fully nonlocal term giving little effect. On the other hand, our results for the van der Waals complexes show that the self-consistency has little effect on the atomic interaction energy and structure at equilibrium distances. This finding validates previous calculations with the same functional that treated the fully nonlocal term as a post-GGA perturbation. A comparison of our results with wave-function calculations demonstrates the usefulness of our approach. The exchange-correlation potential also allows us to calculate Hellmann-Feynman forces, hence providing the means for efficient geometry relaxations as well as unleashing the potential use of other standard techniques that depend on the self-consistent charge distribution. The nature of the van der Waals bond is discussed in terms of the self-consistent bonding charge.

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. Our mapping of the energy required to twist each of the 10 unique base pair steps should provide valuable information for future studies of nucleic acid stability and dynamics. The method introduced will enable the nonempirical theoretical study of significantly larger pieces of DNA or DNA/amino acid complexes than previously possible.

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.

Surface control of optical properties in silicon nanoclusters

Density functional and quantum Monte Carlo calculations are employed to determine the effect of surface passivants on the optical gap of silicon nanoclusters. Our results show that quantum confinement is only one mechanism responsible for visible photoluminescence and that the specific surface chemistry must be taken into account in order to interpret experimental results. Significant changes occur in the optical gap of fully hydrogenated silicon nanoclusters when the surface contains passivants that change the bonding network at the surface. In the case of just one double-bonded oxygen atom, the gap reduction computed as a function of the nanocluster size demonstrates that one contaminant can greatly alter the optical gap. A further significant reduction of the gap occurs with multiple double-bonded oxygen contamination, providing a consistent interpretation of several recent experiments. We predict that other passivants that distort the tetrahedral bonding network at the surface, including other double-bonded groups and in some cases bridged oxygen, will also significantly affect the optical gap. Conversely, single-bonded passivants will have a minimal influence on the optical gap. A discussion of the difference in the strength of the optical transitions for clusters with different passivants is presented.

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. We find that this structural change is connected to a change in interaction from H (water)/pi cloud (benzene) to O (water)/H (benzene). We compare our results for the ground-state structure with results from experiments and quantum-chemical calculations.

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. In addition, we considered two methods for dealing with exchange, namely, using the revPBE generalized gradient functional as well as full Hartree-Fock. Our results are compared with other methods, such as Moller-Plesset and coupled-cluster calculations, thereby suggesting the usefulness of our approach. Since our density functional based method is considerably faster than other standard methods, it provides a computationally inexpensive alternative, which is of particular interest for larger systems where standard calculations are too expensive or infeasible.

Passivation effects 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 impurity passivants such as atomic oxygen. Our results show that quantum confinement is only one mechanism responsible for visible photoluminescence (PL) in silicon nanoclusters and that the specific surface chemistry must be taken into account in order to interpret experimental results. 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.