Bulletin of the American Physical Society
APS March Meeting 2022
Volume 67, Number 3
Monday–Friday, March 14–18, 2022; Chicago
Session W48: Computational Methods for Statistical Mechanics: Advances and Applications IIIFocus Recordings Available

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Sponsoring Units: DCOMP GSNP Chair: Chenxing Luo, Columbia University Room: McCormick Place W471A 
Thursday, March 17, 2022 3:00PM  3:36PM 
W48.00001: Accelerated Monte Carlo sampling through nonreversibility and factorisation Invited Speaker: Manon Michel In the context of the highdimensionality and multimodality of the potentials encountered in chemical physics, massive efforts have been devoted into the development of efficient simulation methodologies. Since the first works of Molecular dynamics (MD) and Markovchain Monte Carlo (MCMC) methods, bidimensional particle systems, in particular with hardcore interactions, have played the role of a challenging but inspiring test bed. In spite of their apparent simplicity, they exhibit a rich behavior where topological defects bind or unbind themselves in pair, ruling by doing so the transition phases. Standard detailedbalance (or reversible) Markov chains then often fail at exploring efficiently the related configuration space. I will first present the MCMC simulation strategies known as EventChain Monte Carlo and based on rejectionfree and nonreversible processes, which were first developed to address this challenge. These methods are based on the breaking of detailed balance in the underlying stochastic sampling processes, upgrading them from discrete Markov chains to piecewise deterministic Markov processes (PDMP). PDMP sampling has now shown performances competing with its MD counterparts in chemical physics and, beyond, with stateoftheart sampling schemes, e.g. Hamiltonian Monte Carlo, in statistical inference. I will discuss how the generalisation of these methods to most systems was made possible by consistently replacing the timereversibility symmetry of detailed balance by potential or global symmetries. I will finally address the question of practical implementation, in particular regarding computational complexity and how the factorisation, first used to design the first PDMP for sphere systems, can be used even in standard MCMC schemes to reduce the computational complexity cost, as e.g. in longrange systems. 
Thursday, March 17, 2022 3:36PM  3:48PM 
W48.00002: Distribution of conductance in disordered mesoscopic systems Kazi Alam, Khandker A Muttalib We present numerical studies of the conductance distribution of disordered mesoscopic conductors based on random matrix theories. These studies explore the dependence of the conductance distribution on different parameters, dimensionality of the system, and disorder. 
Thursday, March 17, 2022 3:48PM  4:00PM 
W48.00003: Choosing the right event (in nonreversible eventchain Monte Carlo) Philipp Hoellmer, Nicolas Noirault, Botao Li, A. C. Maggs, Werner Krauth The general framework of eventchain Monte Carlo (ECMC) constructs nonreversible Markov chains for continuous statisticalphysics models ranging from harddisk systems to longrange interacting molecular systems. Over recent years, several algorithms from the family of ECMC have been proposed, which, in the eventdriven formulation of ECMC, only differ in their treatment of events (that is, e.g., of disk collisions in a harddisk system). Still, we show that different variants can have widely different performances. As a first example, we consider locally stable sparse harddisk packings [1]. Using a scaling theory confirmed by simulation results, we obtain two classes for the escape from slightly relaxed harddisk packings parameterized by a relaxation parameter. In one class, the escape time varies algebraically with the relaxation parameter. In the other class, the escape time only scales as the logarithm of the relaxation parameter. As a second example, we consider integrated autocorrelation times in dense systems of flexible extended harddisk dipoles [2]. Here, the ECMC variants show orderofmagnitude spreads. We expect the performance differences to carry over to longrange interacting molecular systems, where the choice of the optimal ECMC variant is thus highly important. 
Thursday, March 17, 2022 4:00PM  4:12PM 
W48.00004: Axial Nucleus Density for DFT, Symmetry Axis Rotating Toward Electron Clusters with ½, Zeta Related to Two Lockedat180^{o} Dirac Monopoles, Interdimensional Hemispherical (r_{#},θ_{#},φ_{#},z) Relativity Stress Arno Vigen I provide a DFT axial nucleus density to apply that is not in current set.
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Thursday, March 17, 2022 4:12PM  4:24PM Withdrawn 
W48.00005: Improved Estimators for Quantum Typicality Phillip E Weinberg We present improved estimators that can be used along with Quantum Typicality (QT) to reduce statistical noise in thermal expectation values at low temperatures with minimal extra overhead. The estimators use Randomized Lowrank approximations to capture the lowenergy states of the thermal density matrix, $e^{\beta H}$. We demonstrate the efficacy of the improved estimators by looking at the statistical fluctuations in finitetemperature expectation values of a frustrated quantum antiferromagnet. 
Thursday, March 17, 2022 4:24PM  4:36PM 
W48.00006: RejectionFree Population Annealing and Frustrated Ising Models Martin Weigel, Denis Gessert Population annealing (PA) is a sequential Monte Carlo technique suited for massively parallel simulations of systems with sampling diffiulties. While simulating a population of system copies, population control allows the algorithm to amplify well equilibrated copies while getting rid of lesss well adapted ones and hence speed up the overall relaxation process. While this works well for systems with many competing states, it does nothing to speed up sampling in cases where the embedded conventional Markov chain Monte Carlo dynamics is slow due to small acceptance rates, such as for simulations at low temperatures. We show here that a combination of rejectionfree or eventdriven simulation algorithms such as the nfold way with the metaheuristic provided by population annealing allows to successfully tackle problems where low acceptance rates would otherwise make a standard PA simulation inefficient. The method is used to study the behavior of a frustrated Ising model on the honeycomb lattice with ferromagnetic nearestneighbor interactions of strength $J_1$ and antiferromagnetic nextnearestneighbor interactions of strength $J_2$ close to the special point $J_2 = J_1/4$. While conventional methods seemed to indicate slowingdown effects reminiscent of a firstorder phase transition, these are found to be artifacts of the transition temperature approaching zero and the accompanying Metropolis acceptance rates disappearing. In contrast, rejectionfree PA allows to study this problem in detail and map out the phase diagram. 
Thursday, March 17, 2022 4:36PM  4:48PM 
W48.00007: Construction of CRSS model for sapphire with rate theory and molecular dynamic simulations Woo Kyun Kim, Yiyang Du, Dalei Xi, Aditya Nagaraj, Suk Bum Kwon, Sangkee Min Compression test is a popular experiment to study the mechanical behavior of materials during uniaxial loading. In such tests, the critical resolved shear stress (CRSS) is often determined as the characteristic property of material strength. The current work studies CRSS of sapphire under uniaxial compressive stress using molecular dynamic simulations. During each test, a cubic model is subjected to a constant strain rate until plastic deformation is observed, and the CRSS is calculated for each case by applying the Schmid's law. The test is run under various model sizes, strain rates, temperatures, as well as orientations; the CRSS dependencies of these variables are investigated. Furthermore, with the application of rate theory, a mathematical model of CRSS is developed to interpret the variable dependencies. Such a model is fitted to the simulation results to obtain a statistical model. It is found that CRSS of sapphire has a positive relation with strain rate, and negative relations with model size and temperature. For future work, the resulting model can be compared to experimental data, where its effectiveness can be investigated. 
Thursday, March 17, 2022 4:48PM  5:00PM 
W48.00008: Computational studies of the glassforming ability of binary alloys YuanChao Hu, Jan Schroers, Mark D Shattuck, Corey S O'Hern Some alloys easily form glasses, while others do not. The critical cooling rates of good versus poor glassformers can differ by more than 14 orders of magnitude. An important, open problem is to identify the elemental features that give rise to this wide variation in the glassforming ability of alloys. In recent studies, we have performed largescale molecular dynamics simulations of LennardJones and patchyparticle models of alloys to determine how several elemental features, such as the cohesive energy, atomic size ratio, and symmetry of the atomic bonds, affect glass formation in binary alloys. We find that combinations of features that give rise to local icosahedral order possess good glassforming ability. In addition, chemical frustration, characterized by the degree to which the local concentration of different atomic species deviates from that in the liquid state, strongly enhances the glassforming ability. We also identify a new mechanism of “bond shortening” for improving the glassforming ability, where atomic species with weaker repulsive interactions are closer to each other than the other atom types are to each other. These findings for binary alloys can also be used to guide glass design in ternary and multicomponent alloys. 
Thursday, March 17, 2022 5:00PM  5:12PM 
W48.00009: Reversible and irreversible plastic events in a sheared amorphous solid  a mesoscopic approach Dheeraj Kumar, Sylvain Patinet, Muhittin Mungan, Ido Regev

Thursday, March 17, 2022 5:12PM  5:24PM 
W48.00010: PicoScale Ideas for PeopleScale Problems: Leveraging Molecular Simulation Tools for the Modeling of Urban Systems Vincent M Cheng, Kelby B Kramer, Divya Rana, Ryan Rusali, Diana L Warren, Gerald J Wang Techniques from computational science, especially particlebased methods for molecular simulation, have played an enormous role in understanding the physics of amorphous matter. In such simulations, the particles that are amorphously arranged may represent atoms or (in appropriately coarsegrained simulations) molecular/macromolecularscale entities. In this talk, we discuss the power of adapting molecular simulation tools to model amorphous "materials" at the scale of urban phenomena, namely, at the scale of crowds (with particles representing pedestrians) or even at the scale of cities (with particles representing buildings). We argue that the fields of pedestrian dynamics and urbansystems modeling could benefit to a significant extent from this interdisciplinary approach, by leveraging, e.g., the computationally efficient solvers and the descriptors for spatiotemporal correlation popularized by the molecularsimulation community. We provide several representative examples in support of this premise, and close by highlighting future opportunities for synergistic growth between these two communities. 
Thursday, March 17, 2022 5:24PM  5:36PM 
W48.00011: Stereochemistry controlled mechanical properties of crosslinked Poly (Nisopropyl acrylamide) in aqueous medium using molecular simulations sajjad arzemanzadeh, Erfan Norouzi, ehsan hosseinian, Maryam Mahnama Poly (Nisopropyl acrylamide) or PNIPAM is one of the most promising hydrogels with diverse applications in tissue engineering and drug delivery due to biocompatibility and tunability of temperature response close to human body. Density of crosslinker primarily limits water absorption of hydrogels below LCST (swollen state). In this work, we use allatom molecular dynamics to simulate the dynamic bond formation between PNIPAM and its crosslinker to study the influence of stereochemistry and degree of crosslinking (DOC) in aqueous medium in swollen and collapsed states of crosslinked PNIPAM to engineer mechanical properties. Specifically, we obtain radius of gyration (Rg), stressstrain curve, shear modulus (G) and Young's modulus (E) in compression and tension. This study provides a better understanding of customized hydrogel behavior and finetuning of its mechanical properties for different applications. 
Thursday, March 17, 2022 5:36PM  5:48PM 
W48.00012: Not Saved by the Bell: NonGaussianity and Fluid Transport Measurements in MolecularDynamics Simulations Yuanhao Li, Gerald J Wang In interpreting moleculardynamics (MD) simulations, it is frequently taken for granted that measurements meet (seemingly vanilla) standards of statistical regularity, enabling the use of commonplace statistical procedures for model fitting and uncertainty quantification. In this talk, we explore several systems in which such assumptions break down in subtle and surprising ways, with a specific focus on fluid diffusion under spatial confinement. In a wideranging array of MD simulations reflecting systems of engineering interest, we demonstrate that distributions of meansquared displacement exhibit significant nonGaussianity at fixed times as well as significant heteroscedasticity over time. If unaccounted for, these statistical qualities tend to lead to underestimates of the simulation time required to characterize fluid transport and, ultimately, overconfident estimates of transport quantities. Drawing on modern tools for regression analysis of time series, we discuss strategies for ameliorating these issues. Our work highlights the unusual statistical challenges posed by (and numerous opportunities to improve the interpretation of) MD simulations of fluids, especially in confined and heterogeneous environments. 
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