325 research outputs found
Ferroelectricity and isotope effects in hydrogen-bonded KDP crystals
Based on an accurate first principles description of the energetics in
H-bonded KDP, we conduct a first study of nuclear quantum effects and of the
changes brought about by deuteration. Cluster tunneling involving also heavy
ions is allowed, the main effect of deuteration being a depletion of the proton
probability density at the O-H-O bridge center, which in turn weakens its
proton-mediated covalent bonding. The ensuing lattice expansion couples
selfconsistently with the proton off-centering, thus explaining both the giant
isotope effect, and its close connection with geometrical effects.Comment: 4 two-column pages, 4 figure
An ab initio path integral Monte Carlo simulation method for molecules and clusters: application to Li_4 and Li_5^+
A novel method for simulating the statistical mechanics of molecular systems
in which both nuclear and electronic degrees of freedom are treated quantum
mechanically is presented. The scheme combines a path integral description of
the nuclear variables with a first-principles adiabatic description of the
electronic structure. The electronic problem is solved for the ground state
within a density functional approach, with the electronic orbitals expanded in
a localized (Gaussian) basis set. The discretized path integral is computed by
a Metropolis Monte Carlo sampling technique on the normal modes of the
isomorphic ring-polymer. An effective short-time action correct to order
is used. The validity and performance of the method are tested in two
small Lithium clusters, namely Li and Li. Structural and electronic
properties computed within this fully quantum-mechanical scheme are presented
and compared to those obtained within the classical nuclei approximation.
Quantum delocalization effects are significant but tunneling turns out to be
irrelevant at low temperatures.Comment: 11 text pages, 7 figures, to be published in J. Chem. Phy
Molecular effects in the ionization of N, O and F by intense laser fields
In this paper we study the response in time of N, O and F to
laser pulses having a wavelength of 390nm. We find single ionization
suppression in O and its absence in F, in accordance with experimental
results at nm. Within our framework of time-dependent density
functional theory we are able to explain deviations from the predictions of
Intense-Field Many-Body -Matrix Theory (IMST). We confirm the connection of
ionization suppression with destructive interference of outgoing electron waves
from the ionized electron orbital. However, the prediction of ionization
suppression, justified within the IMST approach through the symmetry of the
highest occupied molecular orbital (HOMO), is not reliable since it turns out
that, e.g. in the case of F, the electronic response to the laser pulse is
rather complicated and does not lead to dominant depletion of the HOMO.
Therefore, the symmetry of the HOMO is not sufficient to predict ionization
suppression. However, at least for F, the symmetry of the dominantly
ionized orbital is consistent with the non-suppression of ionization.Comment: 19 pages, 5 figure
Effect of intrinsic defects on the thermal conductivity of PbTe from classical molecular dynamics simulations
Despite being the archetypal thermoelectric material, still today some of the
most exciting advances in the efficiency of these materials are being achieved
by tuning the properties of PbTe. Its inherently low lattice thermal
conductivity can be lowered to its fundamental limit by designing a structure
capable of scattering phonons over a wide range of length scales. Intrinsic
defects, such as vacancies or grain boundaries, can and do play the role of
these scattering sites. Here we assess the effect of these defects by means of
molecular dynamics simulations. For this we purposely parametrize a Buckingham
potential that provides an excellent description of the thermal conductivity of
this material over a wide temperature range. Our results show that intrinsic
point defects and grain boundaries can reduce the lattice conductivity of PbTe
down to a quarter of its bulk value. By studying the size dependence we also
show that typical defect concentrations and grain sizes realized in experiments
normally correspond to the bulk lattice conductivity of pristine PbTe
Inelastic electron injection in a water chain
Irradiation of biological matter triggers a cascade of secondary particles
that interact with their surroundings, resulting in damage. Low-energy
electrons are one of the main secondary species and electron-phonon interaction
plays a fundamental role in their dynamics. We have developed a method to
capture the electron-phonon inelastic energy exchange in real time and have
used it to inject electrons into a simple system that models a biological
environment, a water chain. We simulated both an incoming electron pulse and a
steady stream of electrons and found that electrons with energies just outside
bands of excited molecular states can enter the chain through phonon emission
or absorption. Furthermore, this phonon-assisted dynamical behaviour shows
great sensitivity to the vibrational temperature, highlighting a crucial
controlling factor for the injection and propagation of electrons in water
How well do Car-Parrinello simulations reproduce the Born-Oppenheimer surface ? Theory and Examples
We derive an analytic expression for the average difference between the
forces on the ions in a Car-Parrinello simulation and the forces obtained at
the same ionic positions when the electrons are at their ground state. We show
that for common values of the fictitious electron mass, a systematic bias may
affect the Car-Parrinello forces in systems where the electron-ion coupling is
large. We show that in the limit where the electronic orbitals are rigidly
dragged by the ions the difference between the two dynamics amounts to a
rescaling of the ionic masses, thereby leaving the thermodynamics intact. We
study the examples of crystalline magnesium oxide and crystalline and molten
silicon. We find that for crystalline silicon the errors are very small. For
crystalline MgO the errors are very large but the dynamics can be quite well
corrected within the rigid-ion model. We conclude that it is important to
control the effect of the electron mass parameter on the quantities extracted
from Car-Parrinello simulations.Comment: Submitted to the Journal of Chemical Physic
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