195 research outputs found
Energetics of the AK13 Semi-Local Kohn-Sham Exchange Energy Functional
The recent non-empirical semi-local exchange functional of Armiento and
K\"ummel, the AK13 [PRL 111, 036402 (2013)] incorporates a number of features
reproduced by higher-order theory. The AK13 potential behaves analogously with
the discontinuous jump associated with the derivative discontinuity at integer
particle numbers. Recent works have established that AK13 gives a qualitatively
improved orbital description compared to other semi-local methods, and
reproduces a band structure closer to higher-order theory. However, its
energies and energetics are inaccurate. The present work further investigates
the deficiency in energetics. In addition to AK13 results, we find that
applying the local-density approximation (LDA) non-self-consistently on the
converged AK13 density gives very reasonable energetics with equilibrium
lattice constants and bulk moduli well described across 14 systems. We also
confirm that the attractive orbital features of AK13 are retained even after
full structural relaxation. Hence, the deficient energetics cannot be a result
of the AK13 orbitals having adversely affected the quality of the electron
density compared to that of usual semi-local functionals; an improved orbital
description and good energetics are not in opposition. We also prove that the
non-self-consistent scheme is equivalent to using a single external-potential
dependent functional in an otherwise consistent KS-DFT scheme. Furthermore, our
results also demonstrate that, while an internally consistent KS functional is
presently missing, non-self-consistent LDA on AK13 orbitals works as a
practical non-empirical computational scheme to predict geometries, bulk
moduli, while retaining the band structure features of AK13 at the
computational cost of semi-local DFT.Comment: 7 pages, 4 figure
Quantum oscillations in the kinetic energy density: Gradient corrections from the Airy gas
We derive a closed form expression for the quantum corrections to the kinetic
energy density (KED) in the Thomas-Fermi (TF) limit of a linear potential model
system in three dimensions (the Airy gas). The universality of the expression
is tested numerically in a number of three dimensional model systems: (i)
jellium surfaces, (ii) hydrogen-like potentials, (iii) systems confined by an
harmonic potential in one and (iv) all three dimensions, and (v) a system with
a cosine potential (the Mathieu gas). Our results confirm that the usual
gradient expansion of extended Thomas-Fermi theory (ETF) does not describe the
quantum oscillations for systems that incorporate surface regions where the
electron density drops off to zero. We find that the correction derived from
the Airy gas is universally applicable to relevant spatial regions of systems
of type (i), (ii), and (iv), but somewhat surprisingly not (iii). We discuss
possible implications of our findings to the development of functionals for the
kinetic energy density.Comment: 15 pages, 9 figure
Structural and electronic properties of Li intercalated graphene on SiC(0001)
We investigate the structural and electronic properties of Li-intercalated
monolayer graphene on SiC(0001) using combined angle-resolved photoemission
spectroscopy and first-principles density functional theory. Li intercalates at
room temperature both at the interface between the buffer layer and SiC and
between the two carbon layers. The graphene is strongly -doped due to charge
transfer from the Li atoms and two -bands are visible at the
-point. After heating the sample to 300C, these -bands
become sharp and have a distinctly different dispersion to that of
Bernal-stacked bilayer graphene. We suggest that the Li atoms intercalate
between the two carbon layers with an ordered structure, similar to that of
bulk LiC. An AA-stacking of these two layers becomes energetically
favourable. The -bands around the -point closely resemble the
calculated band structure of a CLiC system, where the intercalated Li
atoms impose a super-potential on the graphene electronic structure that opens
pseudo-gaps at the Dirac points of the two -cones.Comment: 9 pages, 7 figure
ADAQ: Automatic workflows for magneto-optical properties of point defects in semiconductors
Automatic Defect Analysis and Qualification (ADAQ) is a collection of
automatic workflows developed for high-throughput simulations of
magneto-optical properties of point defect in semiconductors. These workflows
handle the vast number of defects by automating the processes to relax the unit
cell of the host material, construct supercells, create point defect clusters,
and execute calculations in both the electronic ground and excited states. The
main outputs are the magneto-optical properties which include zero-phonon
lines, zero-field splitting, and hyperfine coupling parameters. In addition,
the formation energies are calculated. We demonstrate the capability of ADAQ by
performing a complete characterization of the silicon vacancy in silicon
carbide in the polytype 4H (4H-SiC).Comment: Typo corrected in eq. 3, references adde
Identifying Crystal Structures Beyond Known Prototypes from X-ray Powder Diffraction Spectra
The large amount of powder diffraction data for which the corresponding
crystal structures have not yet been identified suggests the existence of
numerous undiscovered, physically relevant crystal structure prototypes. In
this paper, we present a scheme to resolve powder diffraction data into crystal
structures with precise atomic coordinates by screening the space of all
possible atomic arrangements, i.e., structural prototypes, including those not
previously observed, using a pre-trained machine learning (ML) model. This
involves: (i) enumerating all possible symmetry-confined ways in which a given
composition can be accommodated in a given space group, (ii) ranking the
element-assigned prototype representations using energies predicted using Wren
ML model [Sci.\ Adv.\ 8, eabn4117 (2022)], (iii) assigning and perturbing atoms
along the degree of freedom allowed by the Wyckoff positions to match the
experimental diffraction data (iv) validating the thermodynamic stability of
the material using density-functional theory (DFT). An advantage of the
presented method is that it does not rely on a database of previously observed
prototypes and, therefore is capable of finding crystal structures with
entirely new symmetric arrangements of atoms. We demonstrate the workflow on
unidentified XRD spectra from the ICDD database and identify a number of stable
structures, where a majority turns out to be derivable from known prototypes,
but at least two are found to not be part of our prior structural data sets.Comment: 18 pages including citations and supplementary materials, 4 figures;
overall text improvement; revision of some results in Page
A Stimuli-Responsive Nanocomposite for 3D Anisotropic Cell-Guidance and Magnetic Soft Robotics
Stimuli-responsive materials have the potential to enable the generation of new bioinspired devices with unique physicochemical properties and cell-instructive ability. Enhancing biocompatibility while simplifying the production methodologies, as well as enabling the creation of complex constructs, i.e., via 3D (bio)printing technologies, remains key challenge in the field. Here, a novel method is presented to biofabricate cellularized anisotropic hybrid hydrogel through a mild and biocompatible process driven by multiple external stimuli: magnetic field, temperature, and light. A low-intensity magnetic field is used to align mosaic iron oxide nanoparticles (IOPs) into filaments with tunable size within a gelatin methacryloyl matrix. Cells seeded on top or embedded within the hydrogel align to the same axes of the IOPs filaments. Furthermore, in 3D, C2C12 skeletal myoblasts differentiate toward myotubes even in the absence of differentiation media. 3D printing of the nanocomposite hydrogel is achieved and creation of complex heterogeneous structures that respond to magnetic field is demonstrated. By combining the advanced, stimuli-responsive hydrogel with the architectural control provided by bioprinting technologies, 3D constructs can also be created that, although inspired by nature, express functionalities beyond those of native tissue, which have important application in soft robotics, bioactuators, and bionic devices
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Real Time RF Simulator (RTS) and control
The multi-cavity RTS allows LLRF algorithm development and lab testing prior to commissioning with real cavities and cryomodules. The RTS is a valuable tool since it models the functions, errors and disturbances of real RF systems. The advantage of a RTS over an off-line simulator is that it can be implemented on the actual LLRF hardware, on the same FPGA and processor, and run at the same speed of the LLRF control loop. Additionally the RTS can be shared by collaborators who do not have access to RF systems or when the systems are not available to LLRF engineers. The RTS simulator incorporates hardware, firmware and software errors and limitations of a real implementation, which would be hard to identify and time consuming to model in off-line simulations
Global hybrids from the semiclassical atom theory satisfying the local density linear response
We propose global hybrid approximations of the exchange-correlation (XC)
energy functional which reproduce well the modified fourth-order gradient
expansion of the exchange energy in the semiclassical limit of many-electron
neutral atoms and recover the full local density approximation (LDA) linear
response. These XC functionals represent the hybrid versions of the APBE
functional [Phys. Rev. Lett. 106, 186406, (2011)] yet employing an additional
correlation functional which uses the localization concept of the correlation
energy density to improve the compatibility with the Hartree-Fock exchange as
well as the coupling-constant-resolved XC potential energy. Broad energetical
and structural testings, including thermochemistry and geometry, transition
metal complexes, non-covalent interactions, gold clusters and small
gold-molecule interfaces, as well as an analysis of the hybrid parameters, show
that our construction is quite robust. In particular, our testing shows that
the resulting hybrid, including 20\% of Hartree-Fock exchange and named hAPBE,
performs remarkably well for a broad palette of systems and properties, being
generally better than popular hybrids (PBE0 and B3LYP). Semi-empirical
dispersion corrections are also provided.Comment: 12 pages, 4 figure
Database-driven High-Throughput Calculations and Machine Learning Models for Materials Design
This paper reviews past and ongoing efforts in using high-throughput ab-inito
calculations in combination with machine learning models for materials design.
The primary focus is on bulk materials, i.e., materials with fixed, ordered,
crystal structures, although the methods naturally extend into more complicated
configurations. Efficient and robust computational methods, computational
power, and reliable methods for automated database-driven high-throughput
computation are combined to produce high-quality data sets. This data can be
used to train machine learning models for predicting the stability of bulk
materials and their properties. The underlying computational methods and the
tools for automated calculations are discussed in some detail. Various machine
learning models and, in particular, descriptors for general use in materials
design are also covered.Comment: 19 pages, 2 figure
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