Direct coulomb and exchange interaction in artificial atoms

We determine contributions from the direct Coulomb and exchange interactions to the total interaction in artificial semiconductor atoms. We tune the relative strengths of the two interactions and measure them as a function of the number of confined electrons. The electrons tend to have parallel spins when they occupy nearly degenerate single-particle states. We use a magnetic field to adjust the single-particle-state degeneracy, and find that the spin configurations in an arbitrary magnetic field are well explained in terms of two-electron singlet and triplet states.     

Confinement effects on the structure and dynamics of polymer systems from the mesoscale to the nanoscale

In this article, we review some of our recent progress in experimental and simulation methods for generating, characterizing, and modeling polymer microparticles and nanoparticles in a number of polymer and polymer‐blend systems. By using instrumentation developed for probing single fluorescent molecules in micrometer‐sized liquid droplets, we have shown that polymer particles of nearly arbitrary size and composition can be made with a size dispersion that is ultimately limited by the chain length and number distribution within the droplets. Depending on the timescale for solvent evaporation—a tunable parameter in our experiments—the phase separation of otherwise immiscible polymers can be avoided by confinement effects, and homogeneous polymer‐blend microparticles or nanoparticles can be produced. These particles have tunable properties that can be controlled by the simple adjustment of the size of the particle or the relative mass fractions of the polymer components in solution. Physical, optical, and mechanical properties of a variety of microparticles and nanoparticles, differing in size and composition, have been examined with extensive classical molecular dynamics calculations in conjunction with experiments to gain deeper insights into the fundamental nature of their structure, dynamics, and properties. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 1571‐1590, 2005     

Independent component analysis of nanomechanical responses of cantilever arrays

The ability to detect and identify chemical and biological elements in air or liquid environments is of far reaching importance. Performing this task using technology that minimally impacts the perceived environment is the ultimate goal. The development of functionalized cantilever arrays with nanomechanical sensing is an important step towards this goal. This report couples the feature extraction abilities of independent component analysis (ICA) and the classification techniques of neural networks to analyze the signals produced by microcantilever-array-based nanomechanical sensors. The unique capabilities of this analysis unleash the potential of this sensing technology to accurately identify chemical mixtures and concentrations. Furthermore, it is demonstrated that the knowledge of how the sensor array reacts to individual analytes in isolation is sufficient information to decode mixtures of analytes—a substantial benefit, significantly increasing the analytical utility of these sensing devices.     

Computational experiments on the intramolecular energy flow in macromolecules

The microscopic details of the flow of energy in a single chain of polyethylene containing 300 atoms is discussed. The intramolecular dynamics of the polyethylene molecule is studied as a function of CH stretch excitation, temperature, and pressure. The rate of energy flow from CH stretching modes is found to be very rapid and irreversible, occurring on a timescale of less than 0.5 ps at low temperatures, and increases with temperature. A general characteristic two-phase energy flow behavior is observed, where there is initially a very rapid flow (due to the decay of the initial excitation) followed by a slower flow (due to energy redistribution throughout the system). The mechanism for the initial facile energy flow is shown to involve strong resonant pathways. In particular, a CH stretch/HCH bend Fermi (1:2) resonance is shown to dominate the short-time dynamics and facilitates the overall process of energy redistribution. The increase in the rate of energy flow as a function of the backbone temperature is found to be due to the increase in the density of the bath states for energy redistribution which subsequently results in the formation of new low-order resonant interactions (1:1, etc). The long-time dynamics, associated to complete redistribution of the initial CH stretch energy with all of the 894 available vibrational modes, occurs within a time of 2 ps. This timescale corresponds to the time for intramolecular redistribution. A comparison of the intramolecular redistribution time to that of intermolecular redistribution (redistribution in the condensed or solid phase as opposed to a single chain) is also made. A preliminary study of energy flow in a crystal of polyethylene (system containing 19 polyethylene chains) shows that the energy flow exhibits two very different time behaviors. The first is for the intramolecular redistribution as in the single chain study and the second is for intermolecular (chain-to-chain) redistribution. The timescale for intermolecular redistribution is found to be on the order of 0.2 ns at room temperature and pressure, about two orders of magitude larger than the intramolecular timescale.     

The excitation of HF by two polarized lasers and the effect of laser power

A model calculation shows that a larger fraction of an ensemble of hydrogen fluoride molecules absorb energy from two infrared lasers when the lasers are linearly parallel polarized than when they are perpendicular. The ratio of the fractions of absorbing molecules for the parallel configuration to that of the perpendicular configuration is found to be larger at lower laser powers.     

Numerical calculation of eigenvalues for the Schrödinger equation. III

A recently developed method for calculation of eigenvalues is applied to a four coupled oscillator system previously used to test more approximate methods. Analysis is presented to show how the present method scales for systems of two, three, and four coupled oscillator systems.     

Discrete free—free and free—bound spectra from classical trajectories

A calculation of bound—free and free—free transition frequencies and intensities is presented for a well studied model occurring in a collinear scattering of a particle with a harmonic oscillator.     

Normal Coordinate Analysis for Polymer Systems: Capabilities and New Opportunities

Normal coordinate analysis is an important tool in studying the structure, dynamics, and physical properties of polymer systems. In this article the capabilities of normal coordinate analysis (NCA) are explored in some detail. The use of the eigenvalues and eigenvectors from NCA is catalogued for a wide variety of purposes: for assigning or interpreting polymer spectra, for structural determination, for constructing force fields, for computing heat capacity and other thermodynamic properties, and for computing other physical properties. Examples are given for crystals, melts, and amorphous systems. Also described are methods for characterizing the normal mode vectors that are especially useful for larger systems, in which a large amount of data must be analyzed or where visualization or animation fails. Finally, a recently developed method for eliminating negative eigenvalues in systems with tens of thousands of atoms, trajectory averaging, is presented. Also described are several advances in numerical linear algebra for speeding up the diagonalization phase and for computing physical properties without requiring full diagonalization of the Hessian matrix.     

On the determination and ramifications of chaos in many-body systems: A model study of polyethylene

Computational methods for elucidating information related to the dynamical behavior of multidimensional systems are presented and applied in the area of macromolecular dynamics. Results indicate that the dimensionality of chaotic dynamics for a model of polyethylene can only be characterized for a very narrow range of extremely low temperature (0–2 K). These results provide evidence that suggests the dynamics of this system are completely chaotic at temperatures as low as 10 K. The preponderance of increasing density of low frequency floppy vibrational modes in large molecular systems provides a facile mechanism for the onset of global chaos. Ramifications of such global chaos to the accurate modeling and simulation of macromolecular dynamics is discussed.     

Treatment of multibody interactions in molecular simulations of systems with general bond networks

One of the most formidable difficulties in the computer programming of molecular simulations is the sometimes complicated bookkeeping required for keeping track of internal coordinates and their derivatives. A completely general method for keeping track of stretch (two-body), bend (three-body), and torsion, wag, and other four-body interactions for ANY bond network is presented. Computer code using this method for calculating internal coordinates and their derivatives can be used for completely different types of bond networks, no matter how complex, with little or no modification. The method is designed to incorporate recent improved formulas for calculating internal coordinates and their derivatives to ensure the most optimal calculation sequence. © 1997 John Wiley & Sons, Inc. J Comput Chem 18 : 1513–1522, 1997