Structure and dielectric properties of polar fluids with extended dipoles: results from numerical simulations

The strengths and short-comings of the point-dipole model for polar fluids of spherical molecules are illustrated by considering the physically more relevant case of extended dipoles formed by two opposite charges $\pm q$ separated by a distance $d$ (dipole moment $\mu=q d$). Extensive Molecular Dynamics simulations on a high density dipolar fluid are used to analyse the dependence of the pair structure, dielectric constant \eps and dynamics as a function of the ratio $d/\sigma$ (\sig is the molecular diameter), for a fixed dipole moment $\mu$. The point dipole model is found to agree well with the extended dipole model up to d/\sig \simeq 0.3. Beyond that ratio, \eps shows a non-trivial variation with d/\sig. When d/\sig>0.6, a transition is observed towards a hexagonal columnar phase; the corresponding value of the dipole moment, \mu^2/\sig^3 k T=3, is found to be substantially lower than the value of the point dipole required to drive a similar transition.Comment: 10 pages, 11 figures; Paper submitted to Molecular Physic

Strong pressure-energy correlations in van der Waals liquids

Strong correlations between equilibrium fluctuations of the configurational parts of pressure and energy are found in the Lennard-Jones liquid and other simple liquids, but not in hydrogen-bonding liquids like methanol and water. The correlations, that are present also in the crystal and glass phases, reflect an effective inverse power-law repulsive potential dominating fluctuations, even at zero and slightly negative pressure. In experimental data for supercritical Argon, the correlations are found to be approximately 96%. Consequences for viscous liquid dynamics are discussed.Comment: Phys. Rev. Lett., in pres

Mixed quantum-classical dynamics of an amide-I vibrational excitation in a protein a-helix

Adenosine triphosphate sATPd is known to be the main energy currency of the living cell, and is used as a coenzyme to generate energy for many cellular processes through hydrolysis to adenosine diphosphate sADPd,although the mechanism of energy transfer is not well understood. It has been proposed that following hydrolysis of the ATP cofactor bound to a protein, up to two quanta of amide-I vibrational energy are excited and utilized to bring about important structural changes in the protein. To study whether, and how, amide-I vibrational excitations are capable of leading to protein structural changes, we have added components arising from quantum-mechanical amide-I vibrational excitations to the total energy and force terms within a moleculardynamics simulation. This model is applied to helical deca-alanine as a test case to investigate how its dynamics differs in the presence or absence of an amide-I excitation. We find that the presence of an amide-I excitation can bias the structure toward a more helical state

Trans-phonon effects in ultrafast nano-devices

We report a novel phenomenon in carbon nanotube (CNT) based devices, the transphonon effects, which resemble the transonic effects in aerodynamics. It is caused by dissipative resonance of nanotube phonons similar to the radial breathing mode, and subsequent drastic surge of the dragging force on the sliding tube, and multiple phonon barriers are encountered as the intertube sliding velocity reaches critical values. It is found that the transphonon effects can be tuned by applying geometric constraints or varying chirality combinations of the nanotubes

Density scaling as a property of strongly correlating viscous liquids

We address a recent conjecture according to which the relaxation time $\tau$ of a viscous liquid obeys density scaling ($\tau=F(\rho^\gamma/T)$ where $\rho$ is density) if the liquid is ``strongly correlating,'' i.e., has almost 100% correlation between equilibrium virial and potential-energy fluctuations [Pedersen {\it et al.}, PRL {\bf 100}, 011201 (2008)]. Computer simulations of two model liquids - an asymmetric dumbbell model and the Lewis-Wahnstr\"om OTP model - confirm the conjecture and demonstrate that the scaling exponent $\gamma$ can be accurately predicted from equilibrium fluctuations.Comment: 5 pages, 5 figures. Data added for Lewis-Wahnstrom OT

Molecular dynamics simulation of nanocolloidal amorphous silica particles: Part II

Explicit molecular dynamics simulations were applied to a pair of amorphous silica nanoparticles of diameter 3.2 nm immersed in a background electrolyte. Mean forces acting between the pair of silica nanoparticles were extracted at four different background electrolyte concentrations. Dependence of the inter-particle potential of mean force on the separation and the silicon to sodium ratio, as well as on the background electrolyte concentration, are demonstrated. The pH was indirectly accounted for via the ratio of silicon to sodium used in the simulations. The nature of the interaction of the counter-ions with charged silica surface sites (deprotonated silanols) was also investigated. The effect of the sodium double layer on the water ordering was investigated for three Si:Na+ ratios. The number of water molecules trapped inside the nanoparticles was investigated as the Si:Na+ ratio was varied. Differences in this number between the two nanoparticles in the simulations are attributed to differences in the calculated electric dipole moment. The implications of the form of the potentials for aggregation are also discussed.Comment: v1. 33 pages, 7 figures (screen-quality PDF), submitted to J. Chem. Phys v2. 15 pages, 4 tables, 6 figures. Content, author list and title changed; single space

IPEP: The integrated performance evaluation program for the Department of Energy`s Office of Environmental Management

The quality of the analytical data being provided to DOE`s Office of Environmental Management (EM) for environmental restoration activities and the extent to which these data meet the data quality objectives are critical in the decision-making process. One of several quality metrics that can be used in evaluating a laboratory is its performance in performance evaluation (PE) programs. In support of DOE`s environmental restoration and waste management efforts, EM has been charged with developing and implementing a program to assess the performance of participating laboratories. Argonne National Laboratory (ANL) and DOE`s Environmental Measurements Laboratory (EML) and Radiological and Environmental Sciences Laboratory (RESL) have been collaborating on the development and implementation of a comprehensive Integrated Performance Evaluation Program (IPEP) for DOE-wide implementation. The IPEP will use results from existing inorganic, organic, and radiological PE programs when these are available and appropriate for the analytes and matrices being determined for DOE`s EM activities. Existing programs include the U.S. Environmental Protection Agency`s (EPA`s) Contract Laboratory Program (CLP), the Water Supply (WS) and Water Pollution (WP) PE studies for inorganic and organic analytes, and DOE`s Quality Assessment Program (QAP) for radiological analytes. In addition, DOE has begun the development of the Mixed Analyte Performance Evaluation Program (MAPEP) to address the needs of the DOE Complex. These PE programs provide a spectrum of matrices and analytes covering the various inorganic, organic, and low-level radiologic categories found in routine environmental and waste samples. These PE programs already provide some assessment of laboratory performance; IPEP will expand these assessments by evaluating historical performance, as well as results from multiple PE programs, thereby providing an enhanced usage of the PE program information

Long Distance Correlations in Molecular Orientations of Liquid Water and Shape Dependant Hydrophobic Force

Liquid water, at ambient conditions, has short-range density correlations which are well known in literature. Surprisingly, large scale molecular dynamics simulations reveal an unusually long-distance correlation in `longitudinal' part of dipole-dipole orientational correlations. It is non-vanishing even at 75 \AA{} and falls-off exponentially with a correlation length of about 24 \AA{} beyond solvation region. Numerical evidence suggests that the long range nature of dipole-dipole correlation is due to underlying fluctuating network of hydrogen-bonds in the liquid phase. This correlation is shown to give a shape dependant attraction between two hydrophobic surfaces at large distances of separation and the range of this attractive force is in agreement with experiments. In addition it is seen that quadrupolar fluctuations vanish within the first solvation peak (3 \AA{})Comment: 11 pages, 3 figure

The AMANDA Neutrino Telescope and the Indirect Search for Dark Matter

With an effective telescope area of order 10^4 m^2, a threshold of ~50 GeV and a pointing accuracy of 2.5 degrees, the AMANDA detector represents the first of a new generation of high energy neutrino telescopes, reaching a scale envisaged over 25 years ago. We describe its performance, focussing on the capability to detect halo dark matter particles via their annihilation into neutrinos.Comment: Latex2.09, 16 pages, uses epsf.sty to place 15 postscript figures. Talk presented at the 3rd International Symposium on Sources and Detection of Dark Matter in the Universe (DM98), Santa Monica, California, Feb. 199