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Full-Text Articles in Physics

Qsats: Mpi-Driven Quantum Simulations Of Atomic Solids At Zero Temperature, R. Hinde Dec 2010

Qsats: Mpi-Driven Quantum Simulations Of Atomic Solids At Zero Temperature, R. Hinde

Robert Hinde

We describe QSATS, a parallel code for performing variational path integral simulations of the quantum mechanical ground state of monatomic solids. QSATS is designed to treat Boltzmann quantum solids, in which individual atoms are permanently associated with distinguishable crystal lattice sites and undergo large-amplitude zero-point motions around these sites. We demonstrate the capabilities of QSATS by using it to compute the total energy and potential energy of hexagonal close packed solid 4He at the density 4.61421 x 10-3 a0-3.


Pairwise Additive Model For The He-Mgo(100) Interaction, B. Johnson, R. Hinde Dec 2010

Pairwise Additive Model For The He-Mgo(100) Interaction, B. Johnson, R. Hinde

Robert Hinde

We develop a model, based on pairwise additive He-Mg and He-O interactions, for the potential energy of He adsorbates above a rigid MgO(100) surface. The attractive long-range He-Mg and He-O interactions are assumed to have the form C6/r6, with the C6 coefficients determined from atomic data within the context of the Slater-Kirkwood approximation. The repulsive short-range He-Mg and He-O interactions are assumed to have the form Cp/rp, with the exponent p and the Cp coefficients taken as adjustable parameters. We find that for p=9, the Cp coefficients can be ...


Simulating Ch4 Physisorption On Ionic Crystals: Limitations Of An Atomic Partial Charge Model, Philip Stimac, Robert Hinde Dec 2007

Simulating Ch4 Physisorption On Ionic Crystals: Limitations Of An Atomic Partial Charge Model, Philip Stimac, Robert Hinde

Robert Hinde

We use quantum chemical techniques to evaluate the electrostatic and polarization components of the interaction between a rigid CH4 molecule and a lattice of point charges representing the MgO(100) surface. We find that CH4 positioned above Mg adopts an edge-down configuration in which two H atoms are oriented downward towards the MgO(100) surface and point at O ions in the surface layer. The CH4–MgO(100) electrostatic interaction is substantially less favorable (but is still attractive) for the face-down configuration in which three H atoms point downward. Neither configuration is energetically favorable for CH4 molecules positioned above O ...


Three-Body Interactions In Solid Parahydrogen, Robert Hinde Dec 2007

Three-Body Interactions In Solid Parahydrogen, Robert Hinde

Robert Hinde

We use coupled-cluster ab initio methods to evaluate the non-pairwise-additive interactions in clusters of three parahydrogen (pH2) molecules. For acute triangular (pH2)3 geometries that play a prominent role in solid pH2, these interactions lower substantially the trimer’s total interaction energy. Our findings suggest that a widely-used effective pair potential for solid pH2 derives its accuracy from a fortuitous cancellation of errors at small intermolecular distances.


Fpga Acceleration Of A Quantum Monte Carlo Application, Robert Hinde Dec 2007

Fpga Acceleration Of A Quantum Monte Carlo Application, Robert Hinde

Robert Hinde

Quantum Monte Carlo methods enable us to determine the ground-state properties of atomic or molecular clusters. Here, we present a reconfigurable computing architecture using Field Programmable Gate Arrays (FPGAs) to accelerate two computationally intensive kernels of a Quantum Monte Carlo (QMC) application applied to N-body systems. We focus on two key kernels of the QMC application: acceleration of potential energy and wave function calculations. We compare the performance of our application on two reconfigurable platforms. Firstly, we use a dual-processor 2.4 GHz Intel Xeon augmented with two reconfigurable development boards consisting of Xilinx Virtex-II Pro FPGAs. Using this platform ...


Direct Observation Of H2 Binding To A Metal Oxide Surface, J. Z. Larese, T. Arnold, L. Frazier, R. J. Hinde, A. J. Ramirez-Cuesta Dec 2007

Direct Observation Of H2 Binding To A Metal Oxide Surface, J. Z. Larese, T. Arnold, L. Frazier, R. J. Hinde, A. J. Ramirez-Cuesta

Robert Hinde

Inelastic neutron scattering is used to probe the dynamical response of H2 films adsorbed on MgO(100) as a function of film thickness. Concomitant diffraction measurements and a reduced-dimensionality quantum dynamical model provide insight into the molecule-surface interaction potential. At monolayer thickness, the rotational motion is strongly influenced by the surface, so that the molecules behave like quasiplanar rotors. These findings have a direct impact on understanding how molecular hydrogen binds to the surface of materials used in catalytic and storage applications.


Variational Path Integral Simulations Using Discretized Coordinates, Robert Hinde Dec 2005

Variational Path Integral Simulations Using Discretized Coordinates, Robert Hinde

Robert Hinde

We describe a variational path integral simulation algorithm for quantum Monte Carlo studies of many-body systems in which particles are restricted to occupy sites on a regular simple cubic lattice with lattice constant s, and discuss the algorithm’s potential computational benefits. Application of the algorithm to the weakly bound cluster Ne3 shows that accurate coordinate-space observables for this system can be computed using lattice constants as large as s = 0.2 a0.


Dependence Of The H2–H2 Interaction On The Monomer Bond Lengths: Steps Toward An Accurate Ab Initio Estimate, Robert Hinde Dec 2005

Dependence Of The H2–H2 Interaction On The Monomer Bond Lengths: Steps Toward An Accurate Ab Initio Estimate, Robert Hinde

Robert Hinde

We compute the vibrational coupling between two H2 molecules from ab initio quantum chemical calculations of the H2–H2 potential carried out at the full configuration interaction level of theory using the atom-centered aug-cc-pVTZ basis set for hydrogen. We compare the full configuration interaction results with those obtained using two variants of coupled cluster theory and find that a fully iterative treatment of connected triples may be required to estimate the H2–H2 vibrational coupling accurately using coupled cluster theory.


Interaction-Induced Dipole Moment Of The Ar–H2 Dimer: Dependence On The H2 Bond Length, Robert J. Hinde Dec 2005

Interaction-Induced Dipole Moment Of The Ar–H2 Dimer: Dependence On The H2 Bond Length, Robert J. Hinde

Robert Hinde

We present ab initio calculations of the interaction-induced dipole moment of the Ar–H2 van der Waals dimer. The primary focus of our calculations is on the H2 bond length dependence of the dipole moment, which determines the intensities of both the collision-induced H2 = 1 ← 0 fundamental band in gaseous Ar–H2 mixtures and the dopant-induced H2 = 1 ← 0 absorption feature in Ar-doped solid H2 matrices. Our calculations employ large atom-centered basis sets, diffuse bond functions positioned between the two monomers, and a coupled cluster treatment of valence electron correlation; core-valence correlation effects appear ...


Population Size Bias In Descendant-Weighted Diffusion Quantum Monte Carlo Simulations, G. Lee Warren, Robert Hinde Dec 2005

Population Size Bias In Descendant-Weighted Diffusion Quantum Monte Carlo Simulations, G. Lee Warren, Robert Hinde

Robert Hinde

We consider the influence of population size on the accuracy of diffusion quantum Monte Carlo simulations that employ descendant weighting or forward walking techniques to compute expectation values of observables that do not commute with the Hamiltonian. We show that for a simple model system, the d-dimensional isotropic harmonic oscillator, the population size must increase rapidly with d in order to ensure that the simulations produce accurate results. When the population size is too small, expectation values computed using descendant-weighted diffusion quantum Monte Carlo simulations exhibit significant systematic biases.


Vibrational Dependence Of The H2–H2 C6 Coefficients, Robert Hinde Dec 2004

Vibrational Dependence Of The H2–H2 C6 Coefficients, Robert Hinde

Robert Hinde

We use the sum-over-states formalism to compute the imaginary-frequency dipole polarizabilities for H2, as a function of the H–H bond length, at the full configuration interaction level of theory using atom-centered d-aug-cc-pVQZ basis sets. From these polarizabilities, we obtain isotropic and anisotropic C6 dispersion coefficients for a pair of H2 molecules as functions of the two molecules’ bond lengths.


Infrared-Active Vibron Bands Associated With Substitutional Impurities In Solid Parahydrogen, Robert Hinde Dec 2002

Infrared-Active Vibron Bands Associated With Substitutional Impurities In Solid Parahydrogen, Robert Hinde

Robert Hinde

We present a model for the line shapes of infrared-active Q1(0) vibron bands observed in solid parahydrogen doped with low concentrations of spherical substitutional impurities. The line shapes are highly sensitive to the H2 vibrational dependence of the dopant–H2 interaction. When this vibrational dependence is strong, the dopant can trap the infrared-active vibron in its first solvation shell; in this case, the trapped vibron manifests itself in the absorption spectrum as a narrow feature to the red of the pure solid’s vibron band.


Probing Quantum Solvation With Infrared Spectroscopy: Infrared Activity Induced In Solid Parahydrogen By N2 And Ar Dopants, Robert Hinde Dec 2001

Probing Quantum Solvation With Infrared Spectroscopy: Infrared Activity Induced In Solid Parahydrogen By N2 And Ar Dopants, Robert Hinde

Robert Hinde

We present the first high-resolution study of the infrared (IR) absorption spectra of solid parahydrogen matrices containing low concentrations of N2 or Ar impurities. The spectra reveal dopant-induced absorption features that acquire IR activity through short-range isotropic vibrational transition dipole moments arising from dopant–H2 intermolecular interactions. These dopant-induced features provide new insights into the perturbation of the vibron bands of the H2 matrix by chemical impurities,and thus into the physics of solvation in a quantum solid.


The He-Lih Potential Energy Surface Revisited. I. An Interpolated Rigid Rotor Surface, Robert Hinde Dec 1998

The He-Lih Potential Energy Surface Revisited. I. An Interpolated Rigid Rotor Surface, Robert Hinde

Robert Hinde

We reconsider the potential energy surface of the He–LiH system recently examined by Gianturco and co-workers [F. A. Gianturco et al., Chem. Phys. 215, 227 (1997)]. We compute the He–LiH interaction energy at the CCSD(T) level using large correlation consistent atomic basis sets supplemented with bond functions. To capture the severe anisotropy of the He–LiH potential, we interpolate our ab initio points in the angular direction with cubic splines, then expand the splines in terms of Legendre polynomials. The resulting smooth potential surface differs substantially from that of Gianturco et al.; in particular, our attractive He ...