6,280 research outputs found
Free energy reconstruction from steered dynamics without post-processing
Various methods achieving importance sampling in ensembles of nonequilibrium
trajectories enable to estimate free energy differences and, by
maximum-likelihood post-processing, to reconstruct free energy landscapes.
Here, based on Bayes theorem, we propose a more direct method in which a
posterior likelihood function is used both to construct the steered dynamics
and to infer the contribution to equilibrium of all the sampled states. The
method is implemented with two steering schedules. First, using non-autonomous
steering, we calculate the migration barrier of the vacancy in Fe-alpha.
Second, using an autonomous scheduling related to metadynamics and equivalent
to temperature-accelerated molecular dynamics, we accurately reconstruct the
two-dimensional free energy landscape of the 38-atom Lennard-Jones cluster as a
function of an orientational bond-order parameter and energy, down to the
solid-solid structural transition temperature of the cluster and without
maximum-likelihood post-processing.Comment: Accepted manuscript in Journal of Computational Physics, 7 figure
Composite CDMA - A statistical mechanics analysis
Code Division Multiple Access (CDMA) in which the spreading code assignment
to users contains a random element has recently become a cornerstone of CDMA
research. The random element in the construction is particular attractive as it
provides robustness and flexibility in utilising multi-access channels, whilst
not making significant sacrifices in terms of transmission power. Random codes
are generated from some ensemble, here we consider the possibility of combining
two standard paradigms, sparsely and densely spread codes, in a single
composite code ensemble. The composite code analysis includes a replica
symmetric calculation of performance in the large system limit, and
investigation of finite systems through a composite belief propagation
algorithm. A variety of codes are examined with a focus on the high
multi-access interference regime. In both the large size limit and finite
systems we demonstrate scenarios in which the composite code has typical
performance exceeding sparse and dense codes at equivalent signal to noise
ratio.Comment: 23 pages, 11 figures, Sigma Phi 2008 conference submission -
submitted to J.Stat.Mec
Using bijective maps to improve free energy estimates
We derive a fluctuation theorem for generalized work distributions, related
to bijective mappings of the phase spaces of two physical systems, and use it
to derive a two-sided constraint maximum likelihood estimator of their free
energy difference which uses samples from the equilibrium configurations of
both systems. As an application, we evaluate the chemical potential of a dense
Lennard-Jones fluid and study the construction and performance of suitable
maps.Comment: 17 pages, 11 figure
Local-Aggregate Modeling for Big-Data via Distributed Optimization: Applications to Neuroimaging
Technological advances have led to a proliferation of structured big data
that have matrix-valued covariates. We are specifically motivated to build
predictive models for multi-subject neuroimaging data based on each subject's
brain imaging scans. This is an ultra-high-dimensional problem that consists of
a matrix of covariates (brain locations by time points) for each subject; few
methods currently exist to fit supervised models directly to this tensor data.
We propose a novel modeling and algorithmic strategy to apply generalized
linear models (GLMs) to this massive tensor data in which one set of variables
is associated with locations. Our method begins by fitting GLMs to each
location separately, and then builds an ensemble by blending information across
locations through regularization with what we term an aggregating penalty. Our
so called, Local-Aggregate Model, can be fit in a completely distributed manner
over the locations using an Alternating Direction Method of Multipliers (ADMM)
strategy, and thus greatly reduces the computational burden. Furthermore, we
propose to select the appropriate model through a novel sequence of faster
algorithmic solutions that is similar to regularization paths. We will
demonstrate both the computational and predictive modeling advantages of our
methods via simulations and an EEG classification problem.Comment: 41 pages, 5 figures and 3 table
Density-dependence of functional development in spiking cortical networks grown in vitro
During development, the mammalian brain differentiates into specialized
regions with distinct functional abilities. While many factors contribute to
functional specialization, we explore the effect of neuronal density on the
development of neuronal interactions in vitro. Two types of cortical networks,
dense and sparse, with 50,000 and 12,000 total cells respectively, are studied.
Activation graphs that represent pairwise neuronal interactions are constructed
using a competitive first response model. These graphs reveal that, during
development in vitro, dense networks form activation connections earlier than
sparse networks. Link entropy analysis of dense net- work activation graphs
suggests that the majority of connections between electrodes are reciprocal in
nature. Information theoretic measures reveal that early functional information
interactions (among 3 cells) are synergetic in both dense and sparse networks.
However, during later stages of development, previously synergetic
relationships become primarily redundant in dense, but not in sparse networks.
Large link entropy values in the activation graph are related to the domination
of redundant ensembles in late stages of development in dense networks. Results
demonstrate differences between dense and sparse networks in terms of
informational groups, pairwise relationships, and activation graphs. These
differences suggest that variations in cell density may result in different
functional specialization of nervous system tissue in vivo.Comment: 10 pages, 7 figure
Local Chirality of Low-Lying Dirac Eigenmodes and the Instanton Liquid Model
The reasons for using low-lying Dirac eigenmodes to probe the local structure
of topological charge fluctuations in QCD are discussed, and it is pointed out
that the qualitative double-peaked behavior of the local chiral orientation
probability distribution in these modes is necessary, but not sufficient for
dominance of instanton-like fluctuations. The results with overlap Dirac
operator in Wilson gauge backgrounds at lattice spacings ranging from a~0.04 fm
to a~0.12 fm are reported, and it is found that the size and density of local
structures responsible for double-peaking of the distribution are in
disagreement with the assumptions of the Instanton Liquid Model. More
generally, our results suggest that vacuum fluctuations of topological charge
are not effectively dominated by locally quantized (integer-valued) lumps in
QCD.Comment: 29 pages, 13 figures; v2: minor improvements in presentation, results
and conclusions unchanged, version to appear in PR
Absolute FKBP binding affinities obtained via non-equilibrium unbinding simulations
We compute absolute binding affinities for two ligands bound to the FKBP
protein using non-equilibrium unbinding simulations. The methodology is
straight-forward, requiring little or no modification to many modern molecular
simulation packages. The approach makes use of a physical pathway, eliminating
the need for complicated alchemical decoupling schemes. Results of this study
are promising. For the ligands studied here the binding affinities are
typically estimated within less than 4.0 kJ/mol of the target values; and the
target values are within less than 1.0 kJ/mol of experiment. These results
suggest that non-equilibrium simulation could provide a simple and robust means
to estimate protein-ligand binding affinities.Comment: 9 pages, 3 figures (no necessary color). Changes made to methodology
and results between revision
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