109 research outputs found
RevelsMD: Reduced Variance Estimators of the Local Structure in Molecular Dynamics
RevelsMD is a new open source Python library, which uses reduced variance
force sampling based estimators to calculate 3D particle densities and radial
distribution functions from molecular dynamics simulations. This short note
describes the scientific background of the code, its utility and how it fits
within the current zeitgeist in computational chemistry and materials science.Comment: 3 page note describing an open source cod
Accurate Estimation of Diffusion Coefficients and their Uncertainties from Computer Simulation
Self-diffusion coefficients, , are routinely estimated from molecular
dynamics simulations by fitting a linear model to the observed mean-squared
displacements (MSDs) of mobile species. MSDs derived from simulation suffer
from statistical noise, which introduces uncertainty in the resulting estimate
of . An optimal scheme for estimating will minimise this
uncertainty, i.e., will have high statistical efficiency, and will give an
accurate estimate of the uncertainty itself. We present a scheme for estimating
from a single simulation trajectory with high statistical efficiency and
accurately estimating the uncertainty in the predicted value. The statistical
distribution of MSDs observable from a given simulation is modelled as a
multivariate normal distribution using an analytical covariance matrix for an
equivalent system of freely diffusing particles, which we parameterise from the
available simulation data. We then perform Bayesian regression to sample the
distribution of linear models that are compatible with this model multivariate
normal distribution, to obtain a statistically efficient estimate of and
an accurate estimate of the associated statistical uncertainty
Kinisi:Bayesian analysis of mass transport from molecular dynamics simulations
kinisi is a Python package for estimating transport coefficients—e.g., self-diffusion coefficients, ∗—and their corresponding uncertainties from molecular dynamics simulation data. It includes an implementation of the approximate Bayesian regression scheme described in McCluskey etal. (2023), wherein the mean-squared displacement (MSD) of mobile atoms is modelled as a multivariate normal distribution that is parametrised from the input simulation data. kinisi uses Markov-chain Monte Carlo (Foreman-Mackey et al., 2019; Goodman & Weare, 2010) to sample this model multivariate normal distribution to give a posterior distribution of linear model ensemble MSDs that are compatible with the observed simulation data. For each linear ensemble MSD, x(), a corresponding estimate of the diffusion coefficient, ̂∗ is given via the Einstein relation, ̂∗ =1d x() / 6 d where is time. The posterior distribution of compatible model ensemble MSDs calculated by kinisi gives a point estimate for the most probable value of ∗ , given the observed simulation data, and an estimate of the corresponding uncertainty in ̂∗. kinisi also provides equivalent functionality for estimating collective transport coefficients, i.e., jump-diffusion coefficients and ionic conductivities<br/
Phase segregation and nanoconfined fluid O 2 in a lithium-rich oxide cathode
Lithium-rich oxide cathodes lose energy density during cycling due to atomic disordering and nanoscale structural rearrangements, which are both challenging to characterize. Here we resolve the kinetics and thermodynamics of these processes in an exemplar layered Li-rich (Li1.2–xMn0.8O2) cathode using a combined approach of ab initio molecular dynamics and cluster expansion-based Monte Carlo simulations. We identify a kinetically accessible and thermodynamically favourable mechanism to form O2 molecules in the bulk, involving Mn migration and driven by interlayer oxygen dimerization. At the top of charge, the bulk structure locally phase segregates into MnO2-rich regions and Mn-deficient nanovoids, which contain O2 molecules as a nanoconfined fluid. These nanovoids are connected in a percolating network, potentially allowing long-range oxygen transport and linking bulk O2 formation to surface O2 loss. These insights highlight the importance of developing strategies to kinetically stabilize the bulk structure of Li-rich O-redox cathodes to maintain their high energy densities
Overscreening and Underscreening in Solid-Electrolyte Grain Boundary Space-Charge Layers
Polycrystalline solids can exhibit material properties that differ
significantly from those of equivalent single-crystal samples, in part, because
of a spontaneous redistribution of mobile point defects into so-called
space-charge regions adjacent to grain boundaries. The general analytical form
of these space-charge regions is known only in the dilute limit, where
defect-defect correlations can be neglected. Using kinetic Monte Carlo
simulations of a three-dimensional Coulomb lattice gas, we show that
grain-boundary space-charge regions in non-dilute solid electrolytes exhibit
overscreening -- damped oscillatory space-charge profiles -- and underscreening
-- decay lengths that are longer than the corresponding Debye length and that
increase with increasing defect-defect interaction strength. Overscreening and
underscreening are known phenomena in concentrated liquid electrolytes, and the
observation of functionally analogous behaviour in solid electrolyte
space-charge regions suggests that the same underlying physics drives behaviour
in both classes of systems. We therefore expect theoretical approaches
developed to study non-dilute liquid electrolytes to be equally applicable to
future studies of solid electrolytes
A Graphene Surface Force Balance
We report a method for transferring graphene, grown
by chemical vapor deposition, which produces ultraflat graphene
surfaces (root-mean-square roughness of 0.19 nm) free from
polymer residues over macroscopic areas (>1 cm2). The critical
step in preparing such surfaces involves the use of an intermediate
mica template, which itself is atomically smooth. We demonstrate
the compatibility of these model surfaces with the surface force
balance, opening up the possibility of measuring normal and lateral
forces, including friction and adhesion, between two graphene sheets
either in contact or across a liquid medium. The conductivity of the
graphene surfaces allows forces to be measured while controlling the
surface potential. This new apparatus, the graphene surface force
balance, is expected to be of importance to the future understanding
of graphene in applications from lubrication to electrochemical energy storage systems
Anion-polarisation--directed short-range-order in antiperovskite LiFeSO
Short-range ordering in cation-disordered cathodes can have a significant
effect on their electrochemical properties. Here, we characterise the cation
short-range order in the antiperovskite cathode material LiFeSO, using
density functional theory, Monte Carlo simulations, and synchrotron X-ray
pair-distribution-function data. We predict partial short-range
cation-ordering, characterised by favourable OLiFe oxygen coordination
with a preference for polar cis-OLiFe over non-polar
trans-OLiFe configurations. This preference for polar cation
configurations produces long-range disorder, in agreement with experimental
data. The predicted short-range-order preference contrasts with that for a
simple point-charge model, which instead predicts preferential
trans-OLiFe oxygen coordination and corresponding long-range
crystallographic order. The absence of long-range order in LiFeSO can
therefore be attributed to the relative stability of cis-OLiFe and
other non-OLiFe oxygen-coordination motifs. We show that this effect is
associated with the polarisation of oxide and sulfide anions in polar
coordination environments, which stabilises these polar short-range cation
orderings. We propose similar anion-polarisation-directed short-range-ordering
may be present in other heterocationic materials that contain cations with
different formal charges. Our analysis also illustrates the limitations of
using simple point-charge models to predict the structure of cation-disordered
materials, where other factors, such as anion polarisation, may play a critical
role in directing both short- and long-range structural correlations
Aperture synthesis for gravitational-wave data analysis: Deterministic Sources
Gravitational wave detectors now under construction are sensitive to the
phase of the incident gravitational waves. Correspondingly, the signals from
the different detectors can be combined, in the analysis, to simulate a single
detector of greater amplitude and directional sensitivity: in short, aperture
synthesis. Here we consider the problem of aperture synthesis in the special
case of a search for a source whose waveform is known in detail: \textit{e.g.,}
compact binary inspiral. We derive the likelihood function for joint output of
several detectors as a function of the parameters that describe the signal and
find the optimal matched filter for the detection of the known signal. Our
results allow for the presence of noise that is correlated between the several
detectors. While their derivation is specialized to the case of Gaussian noise
we show that the results obtained are, in fact, appropriate in a well-defined,
information-theoretic sense even when the noise is non-Gaussian in character.
The analysis described here stands in distinction to ``coincidence
analyses'', wherein the data from each of several detectors is studied in
isolation to produce a list of candidate events, which are then compared to
search for coincidences that might indicate common origin in a gravitational
wave signal. We compare these two analyses --- optimal filtering and
coincidence --- in a series of numerical examples, showing that the optimal
filtering analysis always yields a greater detection efficiency for given false
alarm rate, even when the detector noise is strongly non-Gaussian.Comment: 39 pages, 4 figures, submitted to Phys. Rev.
- …