We derive here the form for the exact exchange energy density for a density that decays with Gaussian-type behavior at long range. This functional is intermediate between the B88 and the PW91 exchange functionals. Using this modified functional to match the form expected for Gaussian densities, we propose the X3LYP extended functional. We find that X3LYP significantly outperforms Becke three parameter Lee-Yang-Parr (B3LYP) for describing van der Waals and hydrogen bond interactions, while performing slightly better than B3LYP for predicting heats of formation, ionization potentials, electron affinities, proton affinities, and total atomic energies as validated with the extended G2 set of atoms and molecules. Thus X3LYP greatly enlarges the field of applications for density functional theory. In particular the success of X3LYP in describing the water dimer (with Re and De within the error bars of the most accurate determinations) makes it an excellent candidate for predicting accurate ligand-protein and ligand-DNA interactions.
Molecular compounds-comprised of mechanically interlocked components-such as rotaxanes and catenanes can be designed to display readily controllable internal movements of one component with respect to the other. Since theweak noncovalent bonding interactions that contribute to the template-directed synthesis of such compounds live on between the components thereafter, they can be activated such that the components move in either a linear fashion (rotaxanes) or a rotary manner (catenanes). These molecules can be activated by switching the recognition elements off and on between components chemically, electrically, or optically, such that they perform motions reminiscent of the moving parts in macroscopic machines. This review will highlight how the emergence ofthe mechanical bond in chemistry during the last two decades has brought with it a real prospect of integrating a bottom-up approach, based on molecular design and micro- and nanofabrication, to construct molecular electronic devices that store information at very high densities using minimal power. Although most of the research reported in this review on switchable catenanes and rotaxanes has been carried out in the context of solution-phase mechanical processes, recent results demonstrate that relative mechanical movements between the components in interlocked molecules can be stimulated (a) chemically in Langmuir and Langmuir-Blodgett films, (b) electrochemically as self-assembled monolayers on gold, and (c) electronically within the settings of solid-state devices. Not only has reversible, electronically driven switching been observed in devices incorporating a bistable [2]catenane, but a crosspoint random access memory circuit has been fabricated using an amphiphilic, bistable [2]rotaxane. The experiments provide strong evidence that switchable catenanes and rotaxanes operate mechanically in a soft-matter environment and can withstand simple device-processing steps. Studies on single-walled carbon nanotubes used as one of the electrodes in molecular switch tunnel junctions have revealed that interfacial chemical interactions involving electrodes containing carbon, silicon, and oxygen are good choices when carrying out molecular electronics on the class of rotaxane- and catenane-based molecules reported in this review. This conclusion is supported by differential conductance measurements (at 4K) made with single-molecule transistors using the break-junction method. It transpires that the electronic transport properties in such devices are more sensitive to the chemical nature of the molecule-electrode contacts than the details of the molecules' electronic structure away from the contacts. This result has profound implications for molecular electronics and highlights the importance of also considering the molecules and the electrodes as an integrated system. It all adds up to an integrated systems-oriented approach to nanotechnology that finds its inspiration in the transfer of concepts like molecular recognition from the life sciences into materials science and provides a model for how, in principle, to transfer elements of traditional chemistry to technology platforms that are being developed on the nanoscale. Before there can be any serious prospect of a technology, there has to be some good, sound science in the making. Molecular electronics is very much in its infancy and, as such, it can be expected to give rise to a great deal of intellectually stimulating science before it stands half a chance of becoming a viable companion to silicon-based technology.
We use the density functional theory and x-ray and neutron diffraction to investigate the crystal structures and reaction mechanisms of intermediate phases likely to be involved in decomposition of the potential hydrogen storage material LiAlH{sub 4}. First, we explore the decomposition mechanism of monoclinic LiAlH4 into monoclinic Li{sub 3}AlH{sub 6} plus face-centered cubic (fcc) Al and hydrogen. We find that this reaction proceeds through a five-step mechanism with an overall activation barrier of 36.9 kcal/mol. The simulated x ray and neutron diffraction patterns from LiAlH{sub 4} and Li{sub 3}AlH{sub 6} agree well with experimental data. On the other hand, the alternative decomposition of LiAlH{sub 4} into LiAlH2 plus H2 is predicted to be unstable with respect to that through Li{sub 3}AlH{sub 6}. Next, we investigate thermal decomposition of Li{sub 3}AlH{sub 6} into fcc LiH plus Al and hydrogen, occurring through a four-step mechanism with an activation barrier of 17.4 kcal/mol for the rate-limiting step. In the first and second steps, two Li atoms accept two H atoms from AlH{sub 6} to form the stable Li-H-Li-H complex. Then, two sequential H2 desorption steps are followed, which eventually result in fcc LiH plus fcc Al and hydrogen: Li{sub 3}AlH{sub 6}(monoclinic) {yields} 3 LiH(fcc) + Al(fcc) + 3/2 H{sub 2} is endothermic by 15.8 kcal/mol. The dissociation energy of 15.8 kcal/mol per formula unit compares to experimental enthalpies in the range of 9.8-23.9 kcal/mol. Finally, we explore thermal decomposition of LiH, LiH(s) + Al(s) {yields} LiAl(s) + 1/2 H{sub 2}(g) is endothermic by 4.6 kcal/mol. The B32 phase, which we predict as the lowest energy structure for LiAl, shows covalent bond characters in the Al-Al direction. Additionally, we determine that transformation of LiH plus Al into LiAlH is unstable with respect to transformation of LiH through LiAl.
Molecular dynamics calculations are performed to study the effect of deformation sequence and history on the inelastic behavior of copper interfaces on the nanoscale. An asymmetric 45 deg tilt bicrystal interface is examined, representing an idealized high-angle grain boundary interface. The interface model is subjected to three different deformation paths: tension then shear, shear then tension, and combined proportional tension and shear. Analysis shows that path-history dependent material behavior is confined within a finite layer of deformation around the bicrystal interface. The relationships between length scale and interface properties, such as the thickness of the path-history dependent layer and the interface strength, are discussed in detail.
A conforming representation composed of 2D finite elements and finite Fourier series is applied to 3D nonlinear non-ideal magnetohydrodynamics using a semi-implicit time-advance. The self-adjoint semi-implicit operator and variational approach to spatial discretization are synergistic and enable simulation in the extremely stiff conditions found in high temperature plasmas without sacrificing the geometric flexibility needed for modeling laboratory experiments. Growth rates for resistive tearing modes with experimentally relevant Lundquist number are computed accurately with time-steps that are large with respect to the global Alfven time and moderate spatial resolution when the finite elements have basis functions of polynomial degree (p) two or larger. An error diffusion method controls the generation of magnetic divergence error. Convergence studies show that this approach is effective for continuous basis functions with p {ge} 2, where the number of test functions for the divergence control terms is less than the number of degrees of freedom in the expansion for vector fields. Anisotropic thermal conduction at realistic ratios of parallel to perpendicular conductivity (x{parallel}/x{perpendicular}) is computed accurately with p {ge} 3 without mesh alignment. A simulation of tearing-mode evolution for a shaped toroidal tokamak equilibrium demonstrates the effectiveness of the algorithm in nonlinear conditions, and its results are used to verify the accuracy of the numerical anisotropic thermal conduction in 3D magnetic topologies.
Experimental studies have been performed on the velocity-field characteristics of AlGaN/GaN heterostructures. A pulsed voltage input in combination with a four-point measurement was used in a 50 {Omega} environment to determinethe drift velocity of electrons in the two-dimensional electron gas as a function of the applied electric field. These measurements show an apparent saturation velocity near 3.1 x 10{sub 7} cm/s, at a field of 140 kV/cm. A comparison of these studies shows that the experimental velocities are close to previously published simulations based upon Monte Carlo techniques.
Nonuniformities in both sheath electric field and plasma excitation were observed around dissimilar metals placed on a rf electrode. Spatial maps of the rf sheath electric field obtained by laser-induced fluorescence-dip (LIF-dip) spectroscopy show that the sheath structure was a function of the electrode metal. In addition to the electric-field measurements, LIF, optical emission, and Langmuir probe measurements show nonuniform excitation around the dissimilar metals. The degree and spatial extent of the discharge nonuniformities were dependent on discharge conditions and the history of the metal surfaces.
We use quantum mechanics to characterize the structure and current-voltage performance of the Stoddart-Heath rotaxane-based programmable electronic switch. We find that the current when the ring is on the DNP is 37?58 times the current when the ring is on the TTF, in agreement with experiment (ratio of 10?100). This establishes the basis for iterative experimental?theoretical efforts to optimize systems for molecule-based electronics which we illustrate by predicting the effect of adding a group such as CN to the rotaxane.