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21 Jan 2012

Volume 136, Issue 3, Articles (03xxxx)

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J. Chem. Phys. 136, 035101 (2012); http://dx.doi.org/10.1063/1.3671986 (13 pages)

L. Dupuis and Normand Mousseau
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back to top Condensed Phase Dynamics, Structure, and Thermodynamics: Spectroscopy, Reactions, and Relaxation

Molecular dynamics simulations of shock waves in oriented nitromethane single crystals: Plane-specific effects

Lan He, Thomas D. Sewell, and Donald L. Thompson

J. Chem. Phys. 136, 034501 (2012); http://dx.doi.org/10.1063/1.3676727 (13 pages) | Cited 2 times

Online Publication Date: 17 January 2012

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Molecular dynamics simulations of supported shock waves (shock pressure Ps ∼ 15 GPa) propagating along the [110], [011], [101], and [111] directions in crystalline nitromethane initially at T = 200 K were performed using the nonreactive Sorescu-Rice-Thompson force field [D. C. Sorescu, B. M. Rice, and D. L. Thompson, J. Phys. Chem. B 104, 8406 (2000)10.1021/jp000942q]. These simulations, combined with those from a preceding study of shocks propagating along [100], [010], and [001] directions in nitromethane for similar conditions of temperature and shock pressure [L. He, T. D. Sewell, and D. L. Thompson, J. Chem. Phys. 134, 124506 (2011)10.1063/1.3561397], have been used to study the post-shock relaxation phenomena. Shocks along [010] and [101] lead to a crystal-crystal structure transformation. Shocks propagating along [011], [110], [111], [100], and [001] exhibit plane-specific disordering, which was characterized by calculating as functions of time the 1D mean square displacement (MSD), 2D radial distribution function (RDF), and 2D orientation order parameter P2(θ) in orthogonal planes mutually perpendicular to the shock plane; and by calculating as functions of distance behind the shock front the Cartesian components of intermolecular, intramolecular, and total kinetic energies. The 2D RDF results show that the structural disordering for shocks along [100], [110], and [111] is strongly plane-specific; whereas for shocks along [001] and [011], the loss of crystal structural order is almost equivalent in the orthogonal planes perpendicular to the shock plane. Based on the entire set of simulations, there is a trend for the most extensive disordering to occur in the (010) and (math10) planes, less extensive disordering to occur in the (100) plane, and essentially no disordering to occur in the (001) plane. The 2D P2(θ) and 1D MSD profiles show, respectively, that the orientational and translational disordering is plane-specific, which results in the plane-specific structural disordering observed in the 2D RDF. By contrast, the kinetic energy partitioning and redistribution do not exhibit plane specificity, as shown by the similarity of spatial profiles of the Cartesian components of the intermolecular, intramolecular, and total kinetic energies in orthogonal planes perpendicular to the shock plane.
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62.50.Ef Shock wave effects in solids and liquids
64.70.K- Solid-solid transitions
61.66.Hq Organic compounds
81.30.Hd Constant-composition solid-solid phase transformations: polymorphic, massive, and order-disorder

A tunable Fano resonance in silver nanoshell with a spherically anisotropic core

DaJian Wu, ShuMin Jiang, and XiaoJun Liu

J. Chem. Phys. 136, 034502 (2012); http://dx.doi.org/10.1063/1.3676780 (4 pages) | Cited 5 times

Online Publication Date: 17 January 2012

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The influences of the anisotropic permittivity and permeability in inner core on the Fano resonance have been investigated in Ag nanoshell by means of Mie scattering theory. The decreased inner core radius can enhance the coupling between superradiant and subradiant dipole modes and hence a distinct Fano profile. With increasing the tangential permittivity or permeability of inner core, the Fano resonance shows a redshift and the magnitude of Fano profile increases. The variation of Fano resonance with anisotropic permeability of the core is much weaker than that induced by anisotropic permittivity. We further find that the combined action of the increased tangential permittivity and permeability of inner core can induce a significant enhancement of Fano resonance in Ag nanoshell.
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78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
72.25.-b Spin polarized transport
73.22.Lp Collective excitations
77.22.Ch Permittivity (dielectric function)

Many-body effects are essential in a physically motivated CO2 force field

Kuang Yu and J. R. Schmidt

J. Chem. Phys. 136, 034503 (2012); http://dx.doi.org/10.1063/1.3672810 (7 pages) | Cited 3 times

Online Publication Date: 17 January 2012

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We develop a physically motivated many-body force field for CO2, incorporating explicit three-body interactions parameterized on the basis of two- and three-body symmetry adapted perturbation theory (SAPT) calculations. The potential is parameterized consistently with, and builds upon, our successful SAPT-based two-body CO2 model (“Schmidt, Yu, and McDaniel” (SYM) model) [K. Yu, J. G. McDaniel, and J. R. Schmidt, J. Phys Chem B 115, 10054 (2011)10.1021/jp204563n]. We demonstrate that three-body interactions are essential to achieve an accurate description of bulk properties, and that previous two-body models have therefore necessarily exploited large error cancellations to achieve satisfactory results. The resulting three-body model exhibits excellent second/third virial coefficients and bulk properties over the phase diagram, yielding a nearly empirical parameter-free model. We show that this explicit three-body model can be converted into a computationally efficient, density/temperature-dependent two-body model that reduces almost exactly to our prior SYM model in the high-density limit.
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33.20.Tp Vibrational analysis
31.15.xp Perturbation theory

A comparative density functional study of the low pressure phases of solid ZnX, CdX, and HgX: Trends and relativistic effects

S. Biering and P. Schwerdtfeger

J. Chem. Phys. 136, 034504 (2012); http://dx.doi.org/10.1063/1.3675833 (15 pages) | Cited 1 time

Online Publication Date: 18 January 2012

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A comprehensive density functional study of the group 12 chalcogenides has been carried out to study the impact of relativistic effects on the solid-state and electronic structure of the mercury chalcogenides in order to explain their unique behavior compared to the lighter group 12 congeners. For this, we present scalar-relativistic and nonrelativistic density functional calculations for several crystal structures commonly occurring in ZnX, CdX, and HgX (X = S, Se, and Te). The cohesive energies and other ground-state properties (at the zero-temperature limit) are obtained to identify the low-pressure phases and to discuss relativistic effects. Relativistic crucially influences the crystal structure in HgS, an effect less pronounced in the heavier chalcogenides HgSe and HgTe. However, for HgSe and HgTe we find that relativistic effects have a major impact on the electronic structure, where the change upon neglect of relativity goes as far as to the restoration of semiconducting properties.
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71.20.Nr Semiconductor compounds
61.66.Fn Inorganic compounds
71.15.Mb Density functional theory, local density approximation, gradient and other corrections
71.15.Nc Total energy and cohesive energy calculations
71.15.Rf Relativistic effects

A new method of semigrand canonical ensemble to calculate first-order phase transitions for binary mixtures

Yiping Tang

J. Chem. Phys. 136, 034505 (2012); http://dx.doi.org/10.1063/1.3677193 (7 pages) | Cited 1 time

Online Publication Date: 18 January 2012

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First-order phase transitions of binary mixtures at the given pressure (P) and temperature (T) are studied by taking into account the composition fluctuations. Isothermal-isobaric semigrand canonical ensemble is adopted to find the relations among the total number of molecules, the composition fluctuations and Gibbs free energy density. By combining two identical subsystems of mixtures successively, the free energy density is transformed until being stable and its linear segments represent phase transitions. A new method is developed to calculate the phase equilibriums of binary mixtures. The method handles multiple types and number of phase equilibriums at single time and its solutions are physically justified. One example is shown for calculating the phase diagram of binary Lennard-Jones mixture. It demonstrates that the fluctuations of the total number of molecules in mixtures are fundamental behind phase transitions and the van der Waals loops in Gibbs free energy are reasonable.
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64.70.Ja Liquid-liquid transitions
05.40.-a Fluctuation phenomena, random processes, noise, and Brownian motion

Activation energy for a model ferrous-ferric half reaction from transition path sampling

Christof Drechsel-Grau and Michiel Sprik

J. Chem. Phys. 136, 034506 (2012); http://dx.doi.org/10.1063/1.3677195 (11 pages)

Online Publication Date: 18 January 2012

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Activation parameters for the model oxidation half reaction of the classical aqueous ferrous ion are compared for different molecular simulation techniques. In particular, activation free energies are obtained from umbrella integration and Marcus theory based thermodynamic integration, which rely on the diabatic gap as the reaction coordinate. The latter method also assumes linear response, and both methods obtain the activation entropy and the activation energy from the temperature dependence of the activation free energy. In contrast, transition path sampling does not require knowledge of the reaction coordinate and directly yields the activation energy [C. Dellago and P. G. Bolhuis, Mol. Simul. 30, 795 (2004)10.1080/08927020412331294869]. Benchmark activation energies from transition path sampling agree within statistical uncertainty with activation energies obtained from standard techniques requiring knowledge of the reaction coordinate. In addition, it is found that the activation energy for this model system is significantly smaller than the activation free energy for the Marcus model, approximately half the value, implying an equally large entropy contribution.
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82.20.Pm Rate constants, reaction cross sections, and activation energies
82.20.Wt Computational modeling; simulation
65.60.+a Thermal properties of amorphous solids and glasses: heat capacity, thermal expansion, etc.

Bath-induced correlations and relaxation of vibronic dimers

Maxim F. Gelin, Leah Z. Sharp, Dassia Egorova, and Wolfgang Domcke

J. Chem. Phys. 136, 034507 (2012); http://dx.doi.org/10.1063/1.3676063 (13 pages) | Cited 3 times

Online Publication Date: 18 January 2012

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We consider a vibronic dimer bilinearly coupled through its two vibrational monomer modes to two harmonic reservoirs and study, both analytically and numerically, how correlations of the reservoir-induced fluctuations affect dimer relaxation. For reservoirs with fully correlated fluctuations, we derive an exact quantum master equation for the density matrix of the symmetric vibronic dimer. We demonstrate that reservoirs with fully correlated or anticorrelated fluctuations do not allow for complete vibrational relaxation of the dimer due to the existence of decoherence-free subspaces. For reservoirs with partially correlated fluctuations, we establish the existence of three different mechanisms of vibrational relaxation. Weak inter-monomer couplings, as well as predominantly correlated or anticorrelated fluctuations, render two of these mechanisms relatively inefficient, leading to slow decays of the populations and coherences of the dimer density matrix. The analytical results are illustrated and substantiated by numerical studies of the relaxation behavior of photoexcited dimers.
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33.20.Wr Vibronic, rovibronic, and rotation-electron-spin interactions
33.80.Rv Multiphoton ionization and excitation to highly excited states (e.g., Rydberg states)

Intermolecular relaxation in glycerol as revealed by field cycling 1H NMR relaxometry dilution experiments

R. Meier, D. Kruk, J. Gmeiner, and E. A. Rössler

J. Chem. Phys. 136, 034508 (2012); http://dx.doi.org/10.1063/1.3672096 (8 pages) | Cited 4 times

Online Publication Date: 19 January 2012

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1H spin-lattice relaxation rates R1 = 1/T1 have been measured for partly deuterated glycerol-h5 diluted in fully deuterated glycerol-h0 for progressively lower concentrations of glycerol-h5. By means of the field cycling (FC) technique relaxation dispersion data, R1(ω), have been collected for several temperatures in the frequency range of 10 kHz–20 MHz. In order to disclose the spectral shape of the intra- and intermolecular relaxation, extrapolation of the relaxation data to the zero concentration limit has been performed. The paper confirms that the low frequency excess contribution to the total relaxation rate R1(ω) previously reported for several liquids is of intermolecular origin and reflects translational motion, whereas the high-frequency part is attributed to molecular rotation. Thus, intra- and intermolecular relaxation contributions are spectrally separated. The intermolecular relaxation itself contains also a contribution from rotational motion, which is due to non-central positions of the interacting nuclei in the molecule. This eccentricity effect is quantitatively reproduced by treating the intermolecular spectral density as a sum of translational-like (described by the free diffusion model) and rotational-like contributions (described by a Cole-Davidson function). Applying frequency-temperature superposition master curves as well as individual relaxation dispersion data, R1(ω), are analyzed. It is demonstrated that, in spite of the rotational influence, the translational diffusion coefficients, D(T), can be extracted from the 1H relaxation dispersion which gives 1H NMR relaxometry the potential to become a routine technique determining the diffusion coefficient in liquids.
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33.25.+k Nuclear resonance and relaxation
33.70.Jg Line and band widths, shapes, and shifts
34.20.Gj Intermolecular and atom-molecule potentials and forces
33.15.Mt Rotation, vibration, and vibration-rotation constants
33.15.Hp Barrier heights (internal rotation, inversion, rotational isomerism, conformational dynamics)

A dynamical theory of nucleation for colloids and macromolecules

James F. Lutsko

J. Chem. Phys. 136, 034509 (2012); http://dx.doi.org/10.1063/1.3677191 (23 pages) | Cited 3 times

Online Publication Date: 19 January 2012

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A general theory of nucleation for colloids and macromolecules in solution is formulated within the context of fluctuating hydrodynamics. A formalism for the determination of nucleation pathways is developed and stochastic differential equations for the evolution of order parameters are given. The conditions under which the elements of classical nucleation theory are recovered are determined. The theory provides a justification and extension of more heuristic equilibrium approaches based solely on the free energy. It is illustrated by application to the low-concentration/high-concentration transition in globular proteins, where a novel two-step mechanism is identified, where the first step involves the formation of long-wavelength density fluctuations, and the second step is the actual nucleation event occurring within the fluctuation.
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64.60.qe General theory and computer simulations of nucleation
64.60.qj Studies of nucleation in specific substances
87.15.R- Reactions and kinetics
36.20.-r Macromolecules and polymer molecules
82.70.Dd Colloids
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Estimating the conditions for polariton condensation in organic thin-film microcavities

Eric R. Bittner and Carlos Silva

J. Chem. Phys. 136, 034510 (2012); http://dx.doi.org/10.1063/1.3678015 (5 pages)

Online Publication Date: 20 January 2012

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We examine the possibility of observing Bose condensation of a confined two-dimensional polariton gas in an organic quantum well. We deduce a suitable parameterization of a model polynomial Hamiltonian based upon the cavity geometry, the biexciton binding energy, and similar spectroscopic and structural data. By converting the sum-over-states to a semiclassical integration over D-dimensional phase space, we arrive at a principle of correspondence between ideal and non-ideal Bose gases that share a common critical exponent. Using our results, we can calculate the properties for a model cavity containing an anthracene thin film.
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71.36.+c Polaritons (including photon-phonon and photon-magnon interactions)
64.60.F- Equilibrium properties near critical points, critical exponents
03.75.Nt Other Bose-Einstein condensation phenomena
71.35.-y Excitons and related phenomena
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