1,504 research outputs found
Canonical sampling through velocity-rescaling
We present a new molecular dynamics algorithm for sampling the canonical
distribution. In this approach the velocities of all the particles are rescaled
by a properly chosen random factor. The algorithm is formally justified and it
is shown that, in spite of its stochastic nature, a quantity can still be
defined that remains constant during the evolution. In numerical applications
this quantity can be used to measure the accuracy of the sampling. We
illustrate the properties of this new method on Lennard-Jones and TIP4P water
models in the solid and liquid phases. Its performance is excellent and largely
independent on the thermostat parameter also with regard to the dynamic
properties
Tautomeric equilibrium in condensed phases
We present an ab initio molecular dynamics (MD) investigation of the
tautomeric equilibrium for aqueous solutions of glycine and acetone at
realistic experimental conditions. Metadynamics is used to accelerate proton
migration among tautomeric centers. Due to the formation of complex water-ion
structures involved the proton dynamics in the aqueous environment, standard
enhanced sampling approaches may face severe limitations in providing a general
description of the phenomenon. Recently, we developed a set of Collective
Variables (CVs) designed to study protons transfer reactions in complex
condensed systems [Grifoni et al. PNAS, 2019, 116(10), 4054-4057]. In this work
we applied this approach to study proton dissociation dynamics leading to
tautomeric interconversion of biologically and chemically relevant prototypical
systems, namely glycine and acetone in water. Although relatively simple from a
chemical point of view, the results show that even for these small systems
complex reaction pathways and non-trivial conversion dynamics are observed. The
generality of our method allows obtaining these results without providing any
prior information on the dissociation dynamics but only the atomic species that
can exchange protons in the process. Our results agree with literature
estimates and demonstrate the general applicability of this method in the study
of tautomeric reactions
Reconstructing the Density of States by History-Dependent Metadynamics
We present a novel method for the calculation of the energy density of states
D(E) for systems described by classical statistical mechanics. The method
builds on an extension of a recently proposed strategy that allows the free
energy profile of a canonical system to be recovered within a pre-assigned
accuracy,[A. Laio and M. Parrinello, PNAS 2002]. The method allows a good
control over the error on the recovered system entropy. This fact is exploited
to obtain D(E) more efficiently by combining measurements at different
temperatures. The accuracy and efficiency of the method are tested for the
two-dimensional Ising model (up to size 50x50) by comparison with both exact
results and previous studies. This method is a general one and should be
applicable to more realistic model systems
Fish Welfare in Aquaculture: Physiological and Immunological Activities for Diets, Social and Spatial Stress on Mediterranean Aqua Cultured Species
Welfare assessment currently is less well-characterized for aquatic animals and the clas-
sical methodologies used for terrestrial animals are not adequate to improve our knowledge about
fish well-being. Among different approaches, the status of organism responses can be carried out
using different physiological and biochemical tools. Here, we present the state of the art regarding
fish welfare, methodologies, and experimental results with a particular focus on two important
Mediterranean aquaculture species, Sparus aurata and Dicentrarchus labrax. We introduce an approach
using physiological stress-indicators, growth performance and swimming activity to investigate the
effects of the implantation of electronic tags to facilitate the application of telemetry for aquaculture
purposes. The application of telemetry to research on aquatic organisms has expanded recently, and
its utilization needs to be better understood. The mentioned approaches have been discussed for
application in different aquaculture methodologies. Moreover, social stress and territoriality are
relevant factors in the evaluation of gregarious species that may have consequences on the conditions
of animals farmed in captivity. These aspects, that may impair the ability of fish to respond to various
stimuli or negatively influence the flesh quality, here are analysed through behavioural observation,
flanked by the physiological and immunological approach
Accurate sampling using Langevin dynamics
We show how to derive a simple integrator for the Langevin equation and
illustrate how it is possible to check the accuracy of the obtained
distribution on the fly, using the concept of effective energy introduced in a
recent paper [J. Chem. Phys. 126, 014101 (2007)]. Our integrator leads to
correct sampling also in the difficult high-friction limit. We also show how
these ideas can be applied in practical simulations, using a Lennard-Jones
crystal as a paradigmatic case
Metric tensor as the dynamical variable for variable cell-shape molecular dynamics
We propose a new variable cell-shape molecular dynamics algorithm where the
dynamical variables associated with the cell are the six independent dot
products between the vectors defining the cell instead of the nine cartesian
components of those vectors. Our choice of the metric tensor as the dynamical
variable automatically eliminates the cell orientation from the dynamics.
Furthermore, choosing for the cell kinetic energy a simple scalar that is
quadratic in the time derivatives of the metric tensor, makes the dynamics
invariant with respect to the choice of the simulation cell edges. Choosing the
densitary character of that scalar allows us to have a dynamics that obeys the
virial theorem. We derive the equations of motion for the two conditions of
constant external pressure and constant thermodynamic tension. We also show
that using the metric as variable is convenient for structural optimization
under those two conditions. We use simulations for Ar with Lennard-Jones
parameters and for Si with forces and stresses calculated from first-principles
of density functional theory to illustrate the applications of the method.Comment: 10 pages + 6 figures, Latex, to be published in Physical Review
Elastic Constants of Quantum Solids by Path Integral Simulations
Two methods are proposed to evaluate the second-order elastic constants of
quantum mechanically treated solids. One method is based on path-integral
simulations in the (NVT) ensemble using an estimator for elastic constants. The
other method is based on simulations in the (NpT) ensemble exploiting the
relationship between strain fluctuations and elastic constants. The strengths
and weaknesses of the methods are discussed thoroughly. We show how one can
reduce statistical and systematic errors associated with so-called primitive
estimators. The methods are then applied to solid argon at atmospheric
pressures and solid helium 3 (hcp, fcc, and bcc) under varying pressures. Good
agreement with available experimental data on elastic constants is found for
helium 3. Predictions are made for the thermal expectation value of the kinetic
energy of solid helium 3.Comment: 9 pages doublecolumn, 6 figures, submitted to PR
Stacking-fault energies for Ag, Cu, and Ni from empirical tight-binding potentials
The intrinsic stacking-fault energies and free energies for Ag, Cu, and Ni
are derived from molecular-dynamics simulations using the empirical
tight-binding potentials of Cleri and Rosato [Phys. Rev. B 48, 22 (1993)].
While the results show significant deviations from experimental data, the
general trend between the elements remains correct. This allows to use the
potentials for qualitative comparisons between metals with high and low
stacking-fault energies. Moreover, the effect of stacking faults on the local
vibrational properties near the fault is examined. It turns out that the
stacking fault has the strongest effect on modes in the center of the
transverse peak and its effect is localized in a region of approximately eight
monolayers around the defect.Comment: 5 pages, 2 figures, accepted for publication in Phys. Rev.
Nonperturbative Gauge Fixing and Perturbation Theory
We compare the gauge-fixing approach proposed by Jona-Lasinio and Parrinello,
and by Zwanziger (JPLZ) with the standard Fadeev-Popov procedure, and
demonstrate perturbative equality of gauge-invariant quantities, up to
irrelevant terms induced by the cutoff. We also show how a set of local,
renormalizable Feynman rules can be constructed for the JPLZ procedure.Comment: 9 pages, latex, version to appear in Phys. Rev.
- …