108 research outputs found
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
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
Dipole-quadrupole interactions and the nature of phase III of compressed hydrogen
A new class of strongly infrared active structures is identified for phase
III of compressed molecular H2 by constant-pressure ab initio molecular
dynamics and density-functional perturbation calculations. These are planar
quadrupolar structures obtained as a distortion of low-pressure quadrupolar
phases, after they become unstable at about 150 GPa due to a zone-boundary soft
phonon. The nature of the II-III transition and the origin of the IR activity
are rationalized by means of simple electrostatics, as the onset of a
stabilizing dipole-quadrupole interaction.Comment: 4 pages, 3 figures. To appear in Phys. Rev. Let
Performance Modeling and Analysis of a Thermoelectric Building Envelope for Space Heating
To provide energy-efficient space heating and cooling, a thermoelectric building envelope (TBE) embeds thermoelectric devices in building walls. The thermoelectric device in the building envelope can provide active heating and cooling without requiring refrigerant use and energy transport among subsystems. Thus, the TBE system is energy and environmentally friendly. A few studies experimentally investigated the TBE under limited operating conditions, and only simplified models for the commercial thermoelectric module (TEM) were developed to quantify its performance. A holistic approach to optimum system performance is needed for the optimal system design and operation. The study developed a holistic TBE-building system model in Modelica for system simulation and performance analysis. A theoretical model for a single TEM was first established based on energy conversion and thermoelectric principles. Subsequently, a TBE prototype model combining the TEM model was constructed. The prototype model employing a feedback controller was used in a whole building system simulation for a residential house. The system model computed the overall building energy efficiency and dynamic indoor conditions under varying operating conditions. Simulation results indicate the studied TBE system can meet a heating demand to maintain the desired room temperature at 20 °C when the lowest outdoor temperature is at -26.3 degrees C, with a seasonal heating COP near 1.1, demonstrating a better heating performance than electric heaters. It suggests a potential energy-efficient alternative to the traditional natural gas furnaces and electric heaters for space heating
Hybrid Quantum and Classical Mechanical Monte Carlo Simulations of the Interaction of Hydrogen Chloride with Solid Water Clusters
Monte Carlo simulations using a hybrid quantum and classical mechanical
potential were performed for crystal and amorphous-like HCl-water(n) clusters
The subsystem composed by HCl and one water molecule was treated within Density
Functional Theory, and a classical force field was used for the rest of the
system. Simulations performed at 200 K suggest that the energetic feasibility
of HCl dissociation strongly depends on its initial placement within the
cluster. An important degree of ionization occurs only if HCl is incorporated
into the surface. We observe that local melting does not play a crucial role in
the ionization process.Comment: 14 Latex pages with 4 postscript figures, to appear in Chem. Phys.
Let
Performance Modeling and Analysis of a Thermoelectric Building Envelope for Space Heating
To provide energy-efficient space heating and cooling, a thermoelectric building envelope (TBE) embeds thermoelectric devices in building walls. The thermoelectric device in the building envelope can provide active heating and cooling without requiring refrigerant use and energy transport among subsystems. Thus, the TBE system is energy and environmentally friendly. A few studies experimentally investigated the TBE under limited operating conditions, and only simplified models for the commercial thermoelectric module (TEM) were developed to quantify its performance. A holistic approach to optimum system performance is needed for the optimal system design and operation. The study developed a holistic TBE-building system model in Modelica for system simulation and performance analysis. A theoretical model for a single TEM was first established based on energy conversion and thermoelectric principles. Subsequently, a TBE prototype model combining the TEM model was constructed. The prototype model employing a feedback controller was used in a whole building system simulation for a residential house. The system model computed the overall building energy efficiency and dynamic indoor conditions under varying operating conditions. Simulation results indicate the studied TBE system can meet a heating demand to maintain the desired room temperature at 20 °C when the lowest outdoor temperature is at -26.3 degrees C, with a seasonal heating COP near 1.1, demonstrating a better heating performance than electric heaters. It suggests a potential energy-efficient alternative to the traditional natural gas furnaces and electric heaters for space heating
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