Showing papers by "Roberto Car published in 2008"

Raman Spectra of Graphite Oxide and Functionalized Graphene Sheets

Abstract: We investigate Raman spectra of graphite oxide and functionalized graphene sheets with epoxy and hydroxyl groups and Stone−Wales and 5−8−5 defects by first-principles calculations to interpret our experimental results. Only the alternating pattern of single−double carbon bonds within the sp2 carbon ribbons provides a satisfactory explanation for the experimentally observed blue shift of the G band of the Raman spectra relative to graphite. To obtain these single−double bonds, it is necessary to have sp3 carbons on the edges of a zigzag carbon ribbon.

Nuclear quantum effects in water.

Abstract: A path-integral Car-Parrinello molecular dynamics simulation of liquid water and ice is performed. It is found that the inclusion of nuclear quantum effects systematically improves the agreement of first-principles simulations of liquid water with experiment. In addition, the proton momentum distribution is computed utilizing a recently developed open path-integral molecular dynamics methodology. It is shown that these results are in good agreement with experimental data.

Bending properties of single functionalized graphene sheets probed by atomic force microscopy.

Abstract: We probe the bending characteristics of functionalized graphene sheets with the tip of an atomic force microscope. Individual sheets are transformed from a flat into a folded configuration. Sheets can be reversibly folded and unfolded multiple times, and the folding always occurs at the same location. This observation suggests that the folding and bending behavior of the sheets is dominated by pre-existing kink (or even fault) lines consisting of defects and/or functional groups.

Why are water-hydrophobic interfaces charged?

Abstract: We report ab initio molecular dynamics simulations of hydroxide and hydronium ions near a hydrophobic interface, indicating that both ions behave like amphiphilic surfactants that stick to a hydrophobic hydrocarbon surface with their hydrophobic side. We show that this behavior originates from the asymmetry of the molecular charge distribution which makes one end of the ions strongly hydrophobic while the other end is even more hydrophilic than the regular water (H2O) molecules. The effect is more pronounced for the hydroxide than for the hydronium. Our results are consistent with several experimental observations and explain why hydrophobic surfaces in contact with water acquire a net negative charge, a phenomenon that has important implications for biology and polymer science.

Density functional calculations of nanoscale conductance

Abstract: Density functional calculations for the electronic conductance of single molecules are now common. We examine the methodology from a rigorous point of view, discussing where it can be expected to work, and where it should fail. When molecules are weakly coupled to leads, local and gradient-corrected approximations fail, as the Kohn–Sham levels are misaligned. In the weak bias regime, exchange–correlation corrections to the current are missed by the standard methodology. For finite bias, a new methodology for performing calculations can be rigorously derived using an extension of time-dependent current density functional theory from the Schrodinger equation to a master equation.

Role of dipolar correlations in the infrared spectra of water and ice

Abstract: We report simulated infrared (IR) spectra of deuterated water and ice using Car--Parrinello molecular dynamics with maximally localized Wannier functions. Experimental features are accurately reproduced within the harmonic approximation. By decomposing the line shapes in terms of intramolecular and intermolecular dipole correlation functions, we find that short-range intermolecular dynamic charge fluctuations associated to hydrogen bonds are prominent over the entire spectral range. Our analysis reveals the origin of several spectral features and identifies network bending modes in the far IR range.

Band alignment in molecular devices: Influence of anchoring group and metal work function

Abstract: Density functional theory calculations are carried out to investigate the electronic properties of molecular junctions formed by amine- and thiol-terminated alkane monolayers sandwiched between two metal (Au, Ag) electrodes. Based on extensive analysis of molecular monolayers of varying densities, we establish a relationship between the alignment of the molecular energy levels and the interface dipoles, which shows that the band alignment (BA) in the limit of long, isolated chains is independent of the link group and can be computed from a reference system of noninteracting $\text{molecule}+\text{metal}$ molecule metal electrodes. The main difference between the amine and thiol linkers is the effective dipole moment at the contact. This is very large, about 4.5 D, for amine linkers, leading to a strong dependence of the BA on the monolayer density and a slow convergence to the isolated molecule limit. Instead, this convergence is relatively fast for $S$ anchors due to the very small, $\ensuremath{\sim}0.2\text{ }\text{D}$, effective dipoles at the contacts.

Mechanism of H2 production by the [FeFe]H subcluster of di-iron hydrogenases: Implications for abiotic catalysts

Abstract: To explore the possibility that the active center of the di-iron hydrogenases, the [FeFe]H subcluster, can serve by itself as an efficient hydrogen-producing catalyst, we perform comprehensive calc...

Dynamical optimization for partition theory.

Abstract: How to partition a chemical system into its constituent parts is a classic problem of theoretical chemistry. A formally exact solution has recently been developed, partition theory (PT), based on density functional theory [Cohen, M. H.; Wasserman, A. J. Phys. Chem. A 2007, 111, 2229]. PT presents a constrained optimization problem to which the Car-Parrinello (CP) method of electronic structure theory is well suited. We propose here a generalization of the CP method suitable for PT and thereby make way for its practical numerical implementation. We demonstrate that this CP implementation of PT need not increase the complexity of the computation of the system's electronic structure. The scheme provides an exact DFT formulation of, e.g., atoms in molecules theory that is amenable to numerical implementation.