Robert Hinde

Dependence of the H₂-H₂ interaction on the monomer bond lengths: Steps toward an accurate ab initio estimate

Robert Hinde — Fri, 12 Feb 2010 03:15:50 PST

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.

A Hardware-Accelerated Quantum Monte Carlo framework (HAQMC) for N-body systems

Robert Hinde — Fri, 12 Feb 2010 03:13:40 PST

Interest in the study of structural and energetic properties of highly quantum clusters, such as inert gas clusters has motivated the development of a hardware-accelerated framework for Quantum Monte Carlo simulations. In the Quantum Monte Carlo method, the properties of a system of atoms, such as the ground-state energies, are averaged over a number of iterations. Our framework is aimed at accelerating the computations in each iteration of the QMC application by offloading the calculation of properties, namely energy and trial wave function, onto reconfigurable hardware. This gives a user the capability to run simulations for a large number of iterations, thereby reducing the statistical uncertainty in the properties, and for larger clusters. This framework is designed to run on the Cray XD1 high performance reconfigurable computing platform, which exploits the coarse-grained parallelism of the processor along with the fine-grained parallelism of the reconfigurable computing devices available in the form of field-programmable gate arrays. In this paper, we illustrate the functioning of the framework, which can be used to calculate the energies for a model cluster of helium atoms. In addition, we present the capabilities of the framework that allow the user to vary the chemical identities of the simulated atoms.

FPGA acceleration of a quantum Monte Carlo application

Robert Hinde — Fri, 12 Feb 2010 03:11:22 PST

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, we achieve a speedup of 3 over a software-only implementation. Following this, the chemistry application is ported to the Cray XD1 supercomputer equipped with Xilinx Virtex-II Pro and Virtex-4 FPGAs. The hardware-accelerated application on one node of the high performance system equipped with a single Virtex-4 FPGA yields a speedup of approximately 25 over the serial reference code running on one node of the dual-processor dual-core 2.2 GHz AMD Opteron. This speedup is mainly attributed to the use of pipelining, the use of fixedpoint arithmetic for all calculations and the fine-grained parallelism using FPGAs. We can further enhance the performance by operating multiple instances of our design in parallel.

Variational path integral simulations using discretized coordinates

Robert Hinde — Thu, 11 Feb 2010 16:47:33 PST

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.

Probing quantum solvation with infrared spectroscopy: Infrared activity induced in solid parahydrogen by N₂ and Ar dopants

Robert Hinde — Thu, 11 Feb 2010 16:41:41 PST

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.

Three-body interactions in solid parahydrogen

Robert Hinde — Thu, 11 Feb 2010 16:37:11 PST

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.

Population size bias in descendant-weighted diffusion quantum Monte Carlo simulations

Robert Hinde — Thu, 11 Feb 2010 16:32:24 PST

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.

Direct observation of H₂ binding to a metal oxide surface

Robert Hinde — Thu, 11 Feb 2010 16:30:01 PST

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.

Simulating CH₄ physisorption on ionic crystals: Limitations of an atomic partial charge model

Robert Hinde — Thu, 11 Feb 2010 16:27:36 PST

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 ions. We show that for edge-down CH₄ molecules above Mg, the electrostatic component of the CH₄-substrate interaction varies considerably as the CH₄ molecule rotates about the surface normal; the polarization component of the interaction, by contrast, is nearly constant during this rotation. We show that a point-charge model for the CH₄ charge distribution, in which the C and H atoms carry effective partial charges, predicts that the CH₄-surface electrostatic interaction should be more favorable for face-down CH₄ molecules than for edge-down CH₄ molecules, in disagreement with the quantum chemical results. We show that this is because the point-charge model poorly represents the high-order electric multipoles of CH₄.

Infrared-active vibron bands associated with substitutional impurities in solid parahydrogen

Robert Hinde — Thu, 11 Feb 2010 16:22:10 PST

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.

Vibrational dependence of the H₂-H₂ C₆ coefficients

Robert Hinde — Thu, 11 Feb 2010 16:12:56 PST

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.

The He-LiH potential energy surface revisited. I. An interpolated rigid rotor surface

Robert Hinde — Thu, 11 Feb 2010 16:00:47 PST

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-LiH well is more than twice as deep as that of Gianturco et al., with a He-LiH binding energy of De = 176.7 cm-1.

A six-dimensional H₂-H₂ potential energy surface for bound state spectroscopy

Robert Hinde — Thu, 11 Feb 2010 15:40:32 PST

We present a six-dimensional potential energy surface for the (H2)2 dimer based on coupled-cluster electronic structure calculations employing large atom-centered Gaussian basis sets and a small set of midbond functions at the dimer's center of mass. The surface is intended to describe accurately the bound and quasibound states of the dimers (H2)2, (D2)2, and H2-D2 that correlate with H2 or D2 monomers in the rovibrational levels (v, j) =(0,0), (0,2), (1,0), and (1,2). We employ a close-coupled approach to compute the energies of these bound and quasibound dimer states using our potential energy surface, and compare the computed energies for infrared and Raman transitions involving these states with experimentally measured transition energies. We use four of the experimentally measured dimer transition energies to make two empirical adjustments to the ab initio potential energy surface; the adjusted surface gives computed transition energies for 56 experimentally observed transitions that agree with experiment to within 0.036 cm−1. For 26 of the 56 transitions, the agreement between the computed and measured transition energies is within the quoted experimental uncertainty. Finally, we use our potential energy surface to predict the energies of another 34 not-yet-observed infrared and Raman transitions for the three dimers.