111 research outputs found
ASE, GPAW, CMR, and that kind of tools
During the last decades we have developed several, mostly Python-based, tools for setting up, controlling/steering, storing, analyzing, and sharing simulations at the atomic and electronic scale. In particular, we have developed the Atomic Simulation Environment which has become a fairly widely used scripting tool for setting up and controlling simulations with either an interatomic potential code or an electronic structure code as âbackendsâ for the force and energy calculations. More recently, we have developed a real-space density-functional-theory and many-body-perturbation-theory code, GPAW, and most recently the Computational Materials Repository for storing, sharing, and retrieving atomic-scale data. In the discussion, I shall present some of the experiences we have gained through these developments and show some examples of their recent to the problem of finding new materials for efficient solar energy conversion into electricity or fuels
Simulation of Cu-Mg metallic glass: Thermodynamics and Structure
We have obtained effective medium theory (EMT) interatomic potential
parameters suitable for studying Cu-Mg metallic glasses. We present
thermodynamic and structural results from simulations of such glasses over a
range of compositions. We have produced low-temperature configurations by
cooling from the melt at as slow a rate as practical, using constant
temperature and pressure molecular dynamics. During the cooling process we have
carried out thermodynamic analyses based on the temperature dependence of the
enthalpy and its derivative, the specific heat, from which the glass transition
temperature may be determined. We have also carried out structural analyses
using the radial distribution function (RDF) and common neighbor analysis
(CNA). Our analysis suggests that the splitting of the second peak, commonly
associated with metallic glasses, in fact has little to do with the glass
transition itself, but is simply a consequence of the narrowing of peaks
associated with structural features present in the liquid state. In fact the
splitting temperature for the Cu-Cu RDF is well above . The CNA also
highlights a strong similarity between the structure of the intermetallic
alloys and the amorphous alloys of similar composition. We have also
investigated the diffusivity in the supercooled regime. Its temperature
dependence indicates fragile-liquid behavior, typical of binary metallic
glasses. On the other hand, the relatively low specific heat jump of around
indicates apparent strong-liquid behavior, but this can
be explained by the width of the transition due to the high cooling rates.Comment: 12 pages (revtex, two-column), 12 figures, submitted to Phys. Rev.
Spatially resolved quantum plasmon modes in metallic nano-films from first principles
Electron energy loss spectroscopy (EELS) can be used to probe plasmon
excitations in nanostructured materials with atomic-scale spatial resolution.
For structures smaller than a few nanometers quantum effects are expected to be
important, limiting the validity of widely used semi-classical response models.
Here we present a method to identify and compute spatially resolved plasmon
modes from first principles based on a spectral analysis of the dynamical
dielectric function. As an example we calculate the plasmon modes of 0.5-4 nm
thick Na films and find that they can be classified as (conventional) surface
modes, sub-surface modes, and a discrete set of bulk modes resembling standing
waves across the film. We find clear effects of both quantum confinement and
non-local response. The quantum plasmon modes provide an intuitive picture of
collective excitations of confined electron systems and offer a clear
interpretation of spatially resolved EELS spectra.Comment: 7 pages, 7 figure
Plasmons on the edge of MoS2 nanostructures
Using ab initio calculations we predict the existence of one-dimensional
(1D), atomically confined plasmons at the edges of a zigzag MoS2 nanoribbon.
The strongest plasmon originates from a metallic edge state localized on the
sulfur dimers decorating the Mo edge of the ribbon. A detailed analysis of the
dielectric function reveals that the observed deviations from the ideal 1D
plasmon behavior result from single-particle transitions between the metallic
edge state and the valence and conduction bands of the MoS2 sheet. The Mo and S
edges of the ribbon are clearly distinguishable in calculated spatially
resolved electron energy loss spectrum owing to the different plasmonic
properties of the two edges. The edge plasmons could potentially be utilized
for tuning the photocatalytic activity of MoS2 nanoparticles
A local Bayesian optimizer for atomic structures
A local optimization method based on Bayesian Gaussian Processes is developed
and applied to atomic structures. The method is applied to a variety of systems
including molecules, clusters, bulk materials, and molecules at surfaces. The
approach is seen to compare favorably to standard optimization algorithms like
conjugate gradient or BFGS in all cases. The method relies on prediction of
surrogate potential energy surfaces, which are fast to optimize, and which are
gradually improved as the calculation proceeds. The method includes a few
hyperparameters, the optimization of which may lead to further improvements of
the computational speed.Comment: 10 pages, 5 figure
Linear density response function in the projector-augmented wave method: Applications to solids, surfaces, and interfaces
We present an implementation of the linear density response function within
the projector-augmented wave (PAW) method with applications to the linear
optical and dielectric properties of both solids, surfaces, and interfaces. The
response function is represented in plane waves while the single-particle
eigenstates can be expanded on a real space grid or in atomic orbital basis for
increased efficiency. The exchange-correlation kernel is treated at the level
of the adiabatic local density approximation (ALDA) and crystal local field
effects are included. The calculated static and dynamical dielectric functions
of Si, C, SiC, AlP and GaAs compare well with previous calculations. While
optical properties of semiconductors, in particular excitonic effects, are
generally not well described by ALDA, we obtain excellent agreement with
experiments for the surface loss function of the Mg(0001) surface with plasmon
energies deviating by less than 0.2 eV. Finally, we apply the method to study
the influence of substrates on the plasmon excitations in graphene. On
SiC(0001), the long wavelength plasmons are significantly damped although
their energies remain almost unaltered. On Al(111) the plasmon is
completely quenched due to the coupling to the metal surface plasmon.Comment: 11 pages, 8 figures, articl
Unraveling the acoustic electron-phonon interaction in graphene
Using a first-principles approach we calculate the acoustic electron-phonon
couplings in graphene for the transverse (TA) and longitudinal (LA) acoustic
phonons. Analytic forms of the coupling matrix elements valid in the
long-wavelength limit are found to give an almost quantitative description of
the first-principles based matrix elements even at shorter wavelengths. Using
the analytic forms of the coupling matrix elements, we study the acoustic
phonon-limited carrier mobility for temperatures 0-200 K and high carrier
densities of 10^{12}-10^{13} cm^{-2}. We find that the intrinsic effective
acoustic deformation potential of graphene is \Xi_eff = 6.8 eV and that the
temperature dependence of the mobility \mu ~ T^{-\alpha} increases beyond an
\alpha = 4 dependence even in the absence of screening when the full coupling
matrix elements are considered. The large disagreement between our calculated
deformation potential and those extracted from experimental measurements (18-29
eV) indicates that additional or modified acoustic phonon-scattering mechanisms
are at play in experimental situations.Comment: 7 pages, 3 figure
Conventional and acoustic surface plasmons on noble metal surfaces: a time-dependent density functional theory study
First-principles calculations of the conventional and acoustic surface
plasmons (CSPs and ASPs) on the (111) surfaces of Cu, Ag, and Au are presented.
The effect of interband transitions on both types of plasmons is
investigated by comparing results from the local density approximation and an
orbital dependent exchange-correlation (xc) potential that improves the
position and width of the bands. The plasmon dispersions calculated with
the latter xc-potential agree well with electron energy loss spectroscopy
(EELS) experiments. For both the CSP and ASP, the same trend of CuAuAg is
found for the plasmon energies and is attributed to the reduced screening by
interband transitions from Cu, to Au and Ag. This trend for the ASP, however,
contradicts a previous model prediction. While the ASP is seen as a weak
feature in the EELS, it can be clearly identified in the static and dynamic
dielectric band structure.Comment: 5 pages, 4 figure
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