9,370 research outputs found
A comparative study of two molecular mechanics models based on harmonic potentials
We show that the two molecular mechanics models, the stick-spiral and the
beam models, predict considerably different mechanical properties of materials
based on energy equivalence. The difference between the two models is
independent of the materials since all parameters of the beam model are
obtained from the harmonic potentials. We demonstrate this difference for
finite width graphene nanoribbons and a single polyethylene chain comparing
results of the molecular dynamics (MD) simulations with harmonic potentials and
the finite element method with the beam model. We also find that the difference
strongly depends on the loading modes, chirality and width of the graphene
nanoribbons, and it increases with decreasing width of the nanoribbons under
pure bending condition. The maximum difference of the predicted mechanical
properties using the two models can exceed 300% in different loading modes.
Comparing the two models with the MD results of AIREBO potential, we find that
the stick-spiral model overestimates and the beam model underestimates the
mechanical properties in narrow armchair graphene nanoribbons under pure
bending condition.Comment: 40 pages, 21 figure
Ab initio RNA folding
RNA molecules are essential cellular machines performing a wide variety of
functions for which a specific three-dimensional structure is required. Over
the last several years, experimental determination of RNA structures through
X-ray crystallography and NMR seems to have reached a plateau in the number of
structures resolved each year, but as more and more RNA sequences are being
discovered, need for structure prediction tools to complement experimental data
is strong. Theoretical approaches to RNA folding have been developed since the
late nineties when the first algorithms for secondary structure prediction
appeared. Over the last 10 years a number of prediction methods for 3D
structures have been developed, first based on bioinformatics and data-mining,
and more recently based on a coarse-grained physical representation of the
systems. In this review we are going to present the challenges of RNA structure
prediction and the main ideas behind bioinformatic approaches and physics-based
approaches. We will focus on the description of the more recent physics-based
phenomenological models and on how they are built to include the specificity of
the interactions of RNA bases, whose role is critical in folding. Through
examples from different models, we will point out the strengths of
physics-based approaches, which are able not only to predict equilibrium
structures, but also to investigate dynamical and thermodynamical behavior, and
the open challenges to include more key interactions ruling RNA folding.Comment: 28 pages, 18 figure
Efficient construction of free energy profiles of breathing metal–organic frameworks using advanced molecular dynamics simulations
In order to reliably predict and understand the breathing behavior of highly flexible metal–organic frameworks from thermodynamic considerations, an accurate estimation of the free energy difference between their different metastable states is a prerequisite. Herein, a variety of free energy estimation methods are thoroughly tested for their ability to construct the free energy profile as a function of the unit cell volume of MIL-53(Al). The methods comprise free energy perturbation, thermodynamic integration, umbrella sampling, metadynamics, and variationally enhanced sampling. A series of molecular dynamics simulations have been performed in the frame of each of the five methods to describe structural transformations in flexible materials with the volume as the collective variable, which offers a unique opportunity to assess their computational efficiency. Subsequently, the most efficient method, umbrella sampling, is used to construct an accurate free energy profile at different temperatures for MIL-53(Al) from first principles at the PBE+D3(BJ) level of theory. This study yields insight into the importance of the different aspects such as entropy contributions and anharmonic contributions on the resulting free energy profile. As such, this thorough study provides unparalleled insight in the thermodynamics of the large structural deformations of flexible materials
Coupling of Length Scales and Atomistic Simulation of MEMS Resonators
We present simulations of the dynamic and temperature dependent behavior of
Micro-Electro-Mechanical Systems (MEMS) by utilizing recently developed
parallel codes which enable a coupling of length scales. The novel techniques
used in this simulation accurately model the behavior of the mechanical
components of MEMS down to the atomic scale. We study the vibrational behavior
of one class of MEMS devices: micron-scale resonators made of silicon and
quartz. The algorithmic and computational avenue applied here represents a
significant departure from the usual finite element approach based on continuum
elastic theory. The approach is to use an atomistic simulation in regions of
significantly anharmonic forces and large surface area to volume ratios or
where internal friction due to defects is anticipated. Peripheral regions of
MEMS which are well-described by continuum elastic theory are simulated using
finite elements for efficiency. Thus, in central regions of the device, the
motion of millions of individual atoms is simulated, while the relatively large
peripheral regions are modeled with finite elements. The two techniques run
concurrently and mesh seamlessly, passing information back and forth. This
coupling of length scales gives a natural domain decomposition, so that the
code runs on multiprocessor workstations and supercomputers. We present novel
simulations of the vibrational behavior of micron-scale silicon and quartz
oscillators. Our results are contrasted with the predictions of continuum
elastic theory as a function of size, and the failure of the continuum
techniques is clear in the limit of small sizes. We also extract the Q value
for the resonators and study the corresponding dissipative processes.Comment: 10 pages, 10 figures, to be published in the proceedings of DTM '99;
LaTeX with spie.sty, bibtex with spiebib.bst and psfi
Comparative study of semiclassical approaches to quantum dynamics
Quantum states can be described equivalently by density matrices, Wigner
functions or quantum tomograms. We analyze the accuracy and performance of
three related semiclassical approaches to quantum dynamics, in particular with
respect to their numerical implementation. As test cases, we consider the time
evolution of Gaussian wave packets in different one-dimensional geometries,
whereby tunneling, resonance and anharmonicity effects are taken into account.
The results and methods are benchmarked against an exact quantum mechanical
treatment of the system, which is based on a highly efficient Chebyshev
expansion technique of the time evolution operator.Comment: 32 pages, 8 figures, corrected typos and added references; version as
publishe
Molecular dynamics study of solvation effects on acid dissociation in aprotic media
Acid ionization in aprotic media is studied using Molecular Dynamics
techniques. In particular, models for HCl ionization in acetonitrile and
dimethylsulfoxide are investigated. The proton is treated quantum mechanically
using Feynman path integral methods and the remaining molecules are treated
classically. Quantum effects are shown to be essential for the proper treatment
of the ionization. The potential of mean force is computed as a function of the
ion pair separation and the local solvent structure is examined. The computed
dissociation constants in both solvents differ by several orders of magnitude
which are in reasonable agreement with experimental results. Solvent separated
ion pairs are found to exist in dimethylsulfoxide but not in acetonitrile.
Dissociation mechanisms in small clusters are also investigated. Solvent
separated ion pairs persist even in aggregates composed of rather few
molecules, for instance, as few as thirty molecules. For smaller clusters or
for large ion pair separations cluster finite-size effects come into play in a
significant fashion.Comment: Plain LaTeX. To appear in JCP(March 15). Mpeg simulations available
at http://www.chem.utoronto.ca/staff/REK/Videos/clusters/clusters.htm
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