Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session D24: Classical Monte Carlo and Molecular Dynamics Methods |
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Sponsoring Units: DCOMP Room: 203AB |
Monday, March 2, 2015 2:30PM - 2:42PM |
D24.00001: Quantum-Classical Adaptive Coupling in Grand-Canonical like Adaptive Resolution Simulations Animesh Agarwal, Luigi Delle Site We have extended the recently developed Grand Canonical AdResS (GC-AdResS) [1,2] to quantum-classical adaptive coupling where the quantum delocalisation of an atom is described by the path integral formalism. Compared to standard adaptive coupling approaches [3], the advantage of GC-AdResS is that there is no need to obtain a coarse-grained model that correctly reproduces the structural and thermodynamic properties of a full PI (path integral) system, thereby eliminating the need to run a full PI simulation before starting the adaptive simulation. In this context, we have shown that spherical molecules described by a simple generic WCA potential in the coase-grained region, act as a particle reservoir for the PI region. The resulting Grand Canonical set up is such that the structural and dynamical properties of quantum flexible water models in the PI subregion in AdResS are consistent with the properties obtained in the same subregion in full PI simulations.\\[4pt] [1] H. Wang, C. Hartmann, C. Sch\"{u}tte and L. Delle Site, Phys.Rev.X {\bf 3}, 011018 (2013)\\[0pt] [2] A. Agarwal, H. Wang, C. Sch\"{u}tte and L. Delle Site, J.Chem.Phys. {\bf 141}, 034102 (2014) \\[0pt] [3] A.B. Poma and L. Delle Site, Phys. Rev. Lett. {\bf 104}, 250201 (2010) [Preview Abstract] |
Monday, March 2, 2015 2:42PM - 2:54PM |
D24.00002: Efficient cluster Monte Carlo algorithm for Ising spin glasses in more than two space dimensions Andrew J. Ochoa, Zheng Zhu, Helmut G. Katzgraber A cluster algorithm that speeds up slow dynamics in simulations of nonplanar Ising spin glasses away from criticality is urgently needed. In theory, the cluster algorithm proposed by Houdayer poses no advantage over local moves in systems with a percolation threshold below 50\%, such as cubic lattices. However, we show that the frustration present in Ising spin glasses prevents the growth of system-spanning clusters at temperatures roughly below the characteristic energy scale $J$ of the problem. Adding Houdayer cluster moves to simulations of Ising spin glasses for $T \sim J$ produces a speedup that grows with the system size over conventional local moves. We show results for the nonplanar quasi-two-dimensional Chimera graph of the D-Wave Two quantum annealer, as well as conventional three-dimensional Ising spin glasses, where in both cases the addition of cluster moves speeds up thermalization visibly in the physically-interesting low temperature regime. [Preview Abstract] |
Monday, March 2, 2015 2:54PM - 3:06PM |
D24.00003: A new class of scalable parallel pseudorandom number generators based on Pohlig-Hellman exponentiation ciphers Paul Beale We propose a new class of pseudorandom number generators based on Pohlig-Hellman exponentiation ciphers. The method generates uniform pseudorandom streams by encrypting simple sequences of short integer messages into ciphertexts by exponentiation modulo prime numbers. The advantages of the method are: the method is trivially parallelizable by parameterization with each pseudorandom number generator derived from an independent prime modulus, the method is fully scalable on massively parallel computing clusters due to the large number of primes available for each implementation, the seeding and initialization of the independent streams is simple, the method requires only a few integer multiply-mod operations per pseudorandom number, the state of each instance is defined by only a few integer values, the period of each instance is different, and the method passes a battery of intrastream and interstream correlation tests using up to $10^{13}$ pseudorandom numbers per test. We propose an implementation using 32-bit prime moduli with small exponents that require only a few 64-bit multiply-mod operations that can be executed directly in hardware. The 32-bit implementation we propose has millions of possible instances, all with periods greater than $10^{18}$. [Preview Abstract] |
Monday, March 2, 2015 3:06PM - 3:18PM |
D24.00004: Crossover behavior of the thermal conductance and Kramers' transition rate theory Subin Sahu, Kirill Velizhanin, Chih-Chun Chien, Yonatan Dubi, Michael Zwolak Heat transport plays opposing roles in nanotechnology, hindering the miniaturization of electronics on one hand and forming the core of novel heattronic devices on the other. Moreover, heat transport in one-dimensional nanostructures has become a central tool in studying the onset of Fourier's law of heat conduction, a yet unresolved puzzle in theoretical physics. We study the paradigmatic setting of heat transport in one-dimensional systems, a lattice coupled to two heat baths held at different temperatures. Using both numerical and analytical tools, we demonstrate that the heat conductance displays a crossover behavior as the coupling to the thermal reservoirs is tuned. We provide evidence that this behavior is universal by examining harmonic, anharmonic, and disordered systems, and discuss the origin of this effect using an analogy with Kramers' transition state theory for chemical reaction rates. This crossover behavior has important implications in the analysis of numerical results, and suggests a novel way to tune the conductance in nanoscale devices. [Preview Abstract] |
Monday, March 2, 2015 3:18PM - 3:30PM |
D24.00005: Acclerated rare event sampling David Yevick We suggest a strategy for biased transition matrix Monte-Carlo calculations that both ensures the most rapid coverage of the entire computational window in the macroscopic variables of interest $\vec{{E}}$ and yields estimates of transition probabilities between states that are equally accurate in low and high probability regions. Further, paths between different low probability regions are sampled at regular intervals. For the case of a single $E$ variable, a random system realization for which the value of $E$ falls in e.g. the $i$:th histogram bin is generated. This state is perturbed and the resulting realization is rejected until a transition is observed to a neighboring bin, taken here as $i+1$. All accepted and rejected transitions are simultaneously employed to generate the elements of a transition matrix. Subsequently, only a transition to bin $i+2$ is accepted and this procedure is continued until the last of the $N$ bins comprising the computational window is sampled. The procedure is then repeated but in the direction of decreasing bin number. The probability distribution of $E$ can then be obtained by e.g. repeatedly multiplying a random vector by the transition matrix. [Preview Abstract] |
Monday, March 2, 2015 3:30PM - 3:42PM |
D24.00006: The Transition Matrix in Flat-histogram Sampling Gregory Brown, M. Eisenbach, Y. W. Li, G. M. Stocks, D. M. Nicholson, Kh. Odbadrakh, P. A. Rikvold Calculating the thermodynamic density of states (DOS) via flat-histogram sampling is a powerful numerical method for characterizing the temperature-dependent properties of materials. Since the calculated DOS is refined directly from the statistics of the sampling, methods of accelerating the sampling, e.g. through windowing and slow forcing, skew the resulting DOS. Calculating the infinite-temperature transition matrix during the flat-histogram sampling decouples the sampling from estimating the DOS, and allows the techniques of Transition Matrix Monte Carlo to be applied. This enables the calculation of the properties for very large system sizes and thus finite-size scaling analysis of the specific heat, magnetic susceptibility, and cumulant crossings at critical points. We discuss these developments in the context of models for magnetocaloric and spin-crossover materials. This work was performed at the Oak Ridge National Laboratory, which is managed by UT-Battelle for the U.S. Department of Energy. It was sponsored by the U.S. Department of Energy, Office of Basic Energy Sciences, Office of Advanced Scientific Computing Research, and the Oak Ridge Leadership Computing Facility. PAR is supported by the National Science Foundation. [Preview Abstract] |
Monday, March 2, 2015 3:42PM - 3:54PM |
D24.00007: ABSTRACT WITHDRAWN |
Monday, March 2, 2015 3:54PM - 4:06PM |
D24.00008: A More Direct Approach to Finding Phase Transitions in Realistic Alloy Models Derek W. Ostrom, Lance J. Nelson, Conrad W. Rosenbrock, Gus L.W. Hart Cluster expansions provide fast Hamiltonians, allowing for thermodynamic Monte Carlo searches for phase transitions. Monte Carlo simulations often converge slowly and require tuning. A simpler approach may be to use Wang-Landau sampling and compute the transition temperature directly from the partition function. The Wang-Landau algorithm has been successfully demonstrated for toy alloy models. We compare Wang-Landau sampling with thermodynamic Monte Carlo simulations for realistic cluster expansion models with many terms. [Preview Abstract] |
Monday, March 2, 2015 4:06PM - 4:18PM |
D24.00009: Nonstandard Finite-Size Scaling at First-Order Phase Transitions Wolfhard Janke, Marco Mueller, Desmond A. Johnston We show that the standard inverse system volume scaling for finite-size corrections at a first-order phase transition (i.e., $1/L^3$ for an $L \times L \times L$ lattice in $3D$) is transmuted to $1/L^2$ scaling if there is an exponential low-temperature phase degeneracy. The gonihedric Ising model which has a four-spin interaction, plaquette Hamiltonian provides an exemplar of just such a system. We use multicanonical Monte Carlo simulations of this model to generate high-precision data which provides strong confirmation of the nonstandard finite-size scaling law. The dual to the gonihedric model, which is an anisotropically coupled Ashkin-Teller model, has a similar degeneracy and also displays the nonstandard scaling behavior. Further potential applications of the transmuted finite-size scaling law will be briefly discussed.\\[1mm] M. Mueller, W. Janke, D.A. Johnston, Phys. Rev. Lett. {\bf 112} (2014) 200601; M. Mueller, D.A. Johnston, W. Janke, Nucl. Phys. B {\bf 888} (2014) 214. [Preview Abstract] |
Monday, March 2, 2015 4:18PM - 4:30PM |
D24.00010: Nontrivial nonequilibrium critical relaxation in cluster algorithms and universal nonequilibrium-to-equilibrium scaling procedure Yoshihiko Nonomura, Yusuke Tomita Recently we have found that the nonequilibrium relaxation from the perfectly-ordered state of the 2D and 3D Ising models in cluster algorithms shows nontrivial stretched-exponential decay at the transition temperature. Similar nontrivial nonequilibrium critical relaxation is also observed in the 2D XY, 3D XY and 3D Heisenberg models; simple exponential decay in these cases. In order to confirm these behaviors and evaluate the scaling form precisely and robustly, we have proposed a universal scaling procedure to connect nonequilibrium and equilibrium behaviors continuously. For example, when the critical relaxation of the average magnetization $\langle m(t) \rangle$ of a system with linear size $L$ is observed in local-update algorithms, this quantity decays in a power law in the early-stage relaxation with $\langle m(t) \rangle \sim t^{-\beta/(z \nu)}$ and converges to the critical magnetization $m_{\rm c}(L) \sim L^{-\beta/\nu}$ in equilibrium. Then, when $\langle m(t) \rangle L^{\beta/\nu}$ is plotted versus $tL^{-z}$, data for various system sizes are scaled on a single curve in the whole parameter region. This procedure also holds for the cases with cluster algorithms. \smallskip \par \noindent Ref.: Y.~Nonomura, J.\ Phys.\ Soc.\ Jpn. {\bf 83}, 113001 (2014). [Preview Abstract] |
Monday, March 2, 2015 4:30PM - 4:42PM |
D24.00011: Size Effect of YSZ Nanoparticles on Sintering of Ni Nanoparticles in Ni/YSZ Anode of Solid Oxide Fuel Cell via Multi-Nanoparticle Molecular Dynamics Simulation Jingxiang Xu, Yuji Higuchi, Nobuki Ozawa, Momoji Kubo Sintering of Ni nanoparticles leads to the degradation of a Ni/YSZ porous electrode of solid oxide fuel cell. We reported that the YSZ nanoparticle framework plays an important role in inhibition of sintering by using our multi-nanoparticle molecular dynamics simulation method [1]. Size of YSZ nanoparticles affects the framework of YSZ nanoparticles and changes the sintering in Ni/YSZ porous structure. However, the mechanism of different sintering behavior by changing the size of YSZ nanoparticle has not been revealed. In this study, we used our multi-nanoparticle molecular dynamics simulation method to investigate the size effect of YSZ nanoparticles on the sintering of Ni nanoparticles in the Ni/YSZ porous structure. Then, Ni nanoparticles make contact with each other and the sintering proceeds by growth of contact area between Ni nanoparticles when YSZ nanoparticles are large. In contrast, the sintering of Ni nanoparticles is suppressed when YSZ nanoparticles are small. It is found that interfacial area between Ni and YSZ in the small YSZ nanoparticles model is larger than that in the large YSZ nanoparticles model. Thus, the movement of Ni nanoparticles is disturbed, and the sintering is inhibited. [1] J. Xu et al., J. Phys. Chem. C 117 (2013) 9663. [Preview Abstract] |
Monday, March 2, 2015 4:42PM - 4:54PM |
D24.00012: Solvent Repacking Monte Carlo: Application to Phase Behavior of Hard-Disk Mixtures James Kindt An adaptation of configuration bias Monte Carlo called Solvent Repacking Monte Carlo has been developed for grand canonical MC simulation of solutes in condensed phase solutions. This algorithm enables the insertion or removal of a large solute in exchange for a variable number of solvent molecules, which are removed to make room for the solute or ``repacked'' into the solute's cavity. For a proof of concept it has been applied to hard disk mixtures, where the ``solutes'' are disks of radius between 1.4 and 3 times greater than the solvents. The method is efficient enough to allow for the equilibration of the number and spatial distribution of large disks embedded among small disks even above the freezing transition, and allows the estimation of phase boundaries and compositions at coexistence of the fluid and ordered phases in binary hard disk mixtures. The partitioning of large disks between the phases varies non-monotonically with diameter ratio. Possibilities for the applicability of the method to simulations of aqueous solutions using atomistic models will be discussed. [Preview Abstract] |
Monday, March 2, 2015 4:54PM - 5:06PM |
D24.00013: A method for efficient structural simulation of carbon nanostructures and its application to irradiated graphene nanoribbons Colin Daniels, Zachary Bullard, Eduardo Costa Gir\~ao, Vincent Meunier Graphene based nanostructures, including defective ones, are of particular interest for many applications. Due to the inherent stochastic nature of defects in real structures, traditional molecular dynamics approaches are ill-suited for examining average behaviors over time scales relevant to realistic experimental conditions. This talk will introduce and discuss the results of the application of an original Monte-Carlo algorithm for the efficient simulation of the time evolution of carbon nanostructures as they are exposed to external stimuli. I will begin with an overview of the algorithm used, including some simple examples, and move on to an in-depth examination of the application of this algorithm to graphene nanoribbons. In particular, I will examine the atomistic restructuring of different types of carbon nanoribbons as they are irradiated and subjected to uniaxial stress, and discuss the novel properties that emerged thereof. Additionally, the examination will shed light on the use of this method in practice and highlight the utility of other methods, used in conjunction with this algorithm, that allowed for fast prototyping of the system's electronic and magnetic properties. [Preview Abstract] |
Monday, March 2, 2015 5:06PM - 5:18PM |
D24.00014: ABSTRACT WITHDRAWN |
Monday, March 2, 2015 5:18PM - 5:30PM |
D24.00015: Molecular dynamics simulations of nanoscale Al structures for energetic formulations N. Scott Weingarten, Michael Zachariah The addition of metal microparticles, such as aluminum, to molecular explosives results in an increase in the heat of explosion, as well as higher temperatures. It is expected that the use of Al nanoparticles would further enhance these effects, but this has never been realized due to sintering of the nano-Al just prior to the energy release step. Recently, a capability emerged to produce macroscopic quantities of aluminum-based nanoclusters comprising near metallic state Al cores, passivated with an energetic gas generator. We are pursuing the possibility that these nanoclusters can be embedded in a mesoscopic spherical architecture which, upon heating, will lead to the decomposition of the gas generator. This would drive a volumetric expansion that liberates the aluminum nanoclusters which can subsequently undergo exothermic reaction. Atomistic simulations are used to explore the feasibility of this process, and determine the dynamics driving the ejection of the nano-Al particles. [Preview Abstract] |
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