10,458 research outputs found
Exploration of Reaction Pathways and Chemical Transformation Networks
For the investigation of chemical reaction networks, the identification of
all relevant intermediates and elementary reactions is mandatory. Many
algorithmic approaches exist that perform explorations efficiently and
automatedly. These approaches differ in their application range, the level of
completeness of the exploration, as well as the amount of heuristics and human
intervention required. Here, we describe and compare the different approaches
based on these criteria. Future directions leveraging the strengths of chemical
heuristics, human interaction, and physical rigor are discussed.Comment: 48 pages, 4 figure
Interactive Chemical Reactivity Exploration
Elucidating chemical reactivity in complex molecular assemblies of a few
hundred atoms is, despite the remarkable progress in quantum chemistry, still a
major challenge. Black-box search methods to find intermediates and
transition-state structures might fail in such situations because of the
high-dimensionality of the potential energy surface. Here, we propose the
concept of interactive chemical reactivity exploration to effectively introduce
the chemist's intuition into the search process. We employ a haptic pointer
device with force-feedback to allow the operator the direct manipulation of
structures in three dimensions along with simultaneous perception of the
quantum mechanical response upon structure modification as forces. We elaborate
on the details of how such an interactive exploration should proceed and which
technical difficulties need to be overcome. All reactivity-exploration concepts
developed for this purpose have been implemented in the Samson programming
environment.Comment: 36 pages, 14 figure
A Robust and Efficient Method for Solving Point Distance Problems by Homotopy
The goal of Point Distance Solving Problems is to find 2D or 3D placements of
points knowing distances between some pairs of points. The common guideline is
to solve them by a numerical iterative method (\emph{e.g.} Newton-Raphson
method). A sole solution is obtained whereas many exist. However the number of
solutions can be exponential and methods should provide solutions close to a
sketch drawn by the user.Geometric reasoning can help to simplify the
underlying system of equations by changing a few equations and triangularizing
it.This triangularization is a geometric construction of solutions, called
construction plan. We aim at finding several solutions close to the sketch on a
one-dimensional path defined by a global parameter-homotopy using a
construction plan. Some numerical instabilities may be encountered due to
specific geometric configurations. We address this problem by changing
on-the-fly the construction plan.Numerical results show that this hybrid method
is efficient and robust
Monte Carlo evaluation of the equilibrium isotope effects using the Takahashi-Imada factorization of the Feynman path integral
The Feynman path integral approach for computing equilibrium isotope effects
and isotope fractionation corrects the approximations made in standard methods,
although at significantly increased computational cost. We describe an
accelerated path integral approach based on three ingredients: the fourth-
order Takahashi-Imada factorization of the path integral, thermodynamic
integration with respect to mass, and centroid virial estimators for relevant
free energy derivatives. While the frst ingredient speeds up convergence to the
quantum limit, the second and third improve statistical convergence. The
combined method is applied to compute the equilibrium constants for isotope
exchange reactions H2+D=H+HD and H2+D2=2HD
Coarse Molecular Dynamics of a Peptide Fragment: Free Energy, Kinetics, and Long-Time Dynamics Computations
We present a ``coarse molecular dynamics'' approach and apply it to studying
the kinetics and thermodynamics of a peptide fragment dissolved in water. Short
bursts of appropriately initialized simulations are used to infer the
deterministic and stochastic components of the peptide motion parametrized by
an appropriate set of coarse variables. Techniques from traditional numerical
analysis (Newton-Raphson, coarse projective integration) are thus enabled;
these techniques help analyze important features of the free-energy landscape
(coarse transition states, eigenvalues and eigenvectors, transition rates,
etc.). Reverse integration of (irreversible) expected coarse variables backward
in time can assist escape from free energy minima and trace low-dimensional
free energy surfaces. To illustrate the ``coarse molecular dynamics'' approach,
we combine multiple short (0.5-ps) replica simulations to map the free energy
surface of the ``alanine dipeptide'' in water, and to determine the ~ 1/(1000
ps) rate of interconversion between the two stable configurational basins
corresponding to the alpha-helical and extended minima.Comment: The article has been submitted to "The Journal of Chemical Physics.
Computational analysis of single rising bubbles influenced by soluble surfactant
This paper presents novel insights about the influence of soluble surfactants
on bubble flows obtained by Direct Numerical Simulation (DNS). Surfactants are
amphiphilic compounds which accumulate at fluid interfaces and significantly
modify the respective interfacial properties, influencing also the overall
dynamics of the flow. With the aid of DNS local quantities like the surfactant
distribution on the bubble surface can be accessed for a better understanding
of the physical phenomena occurring close to the interface. The core part of
the physical model consists in the description of the surfactant transport in
the bulk and on the deformable interface. The solution procedure is based on an
Arbitrary Lagrangian-Eulerian (ALE) Interface-Tracking method. The existing
methodology was enhanced to describe a wider range of physical phenomena. A
subgrid-scale (SGS) model is employed in the cases where a fully resolved DNS
for the species transport is not feasible due to high mesh resolution
requirements and, therefore, high computational costs. After an exhaustive
validation of the latest numerical developments, the DNS of single rising
bubbles in contaminated solutions is compared to experimental results. The full
velocity transients of the rising bubbles, especially the contaminated ones,
are correctly reproduced by the DNS. The simulation results are then studied to
gain a better understanding of the local bubble dynamics under the effect of
soluble surfactant. One of the main insights is that the quasi-steady state of
the rise velocity is reached without ad- and desorption being necessarily in
local equilibrium
Multiple-scattering effects on incoherent neutron scattering in glasses and viscous liquids
Incoherent neutron scattering experiments are simulated for simple dynamic
models: a glass (with a smooth distribution of harmonic vibrations) and a
viscous liquid (described by schematic mode-coupling equations). In most
situations multiple scattering has little influence upon spectral
distributions, but it completely distorts the wavenumber-dependent amplitudes.
This explains an anomaly observed in recent experiments
Non-adiabatic effects during the dissociative adsorption of O2 at Ag(111)? A first-principles divide and conquer study
We study the gas-surface dynamics of O2 at Ag(111) with the particular
objective to unravel whether electronic non-adiabatic effects are contributing
to the experimentally established inertness of the surface with respect to
oxygen uptake. We employ a first-principles divide and conquer approach based
on an extensive density-functional theory mapping of the adiabatic potential
energy surface (PES) along the six O2 molecular degrees of freedom. Neural
networks are subsequently used to interpolate this grid data to a continuous
representation. The low computational cost with which forces are available from
this PES representation allows then for a sufficiently large number of
molecular dynamics trajectories to quantitatively determine the very low
initial dissociative sticking coefficient at this surface. Already these
adiabatic calculations yield dissociation probabilities close to the scattered
experimental data. Our analysis shows that this low reactivity is governed by
large energy barriers in excess of 1.1 eV very close to the surface.
Unfortunately, these adiabatic PES characteristics render the dissociative
sticking a rather insensitive quantity with respect to a potential spin or
charge non-adiabaticity in the O2-Ag(111) interaction. We correspondingly
attribute the remaining deviations between the computed and measured
dissociation probabilities primarily to unresolved experimental issues with
respect to surface imperfections.Comment: 18 pages including 6 figure
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