105 research outputs found
Machine Learning Energies of 2 M Elpasolite (ABCD) Crystals
Elpasolite is the predominant quaternary crystal structure (AlNaKF
prototype) reported in the Inorganic Crystal Structure Database. We have
developed a machine learning model to calculate density functional theory
quality formation energies of all 2 M pristine ABCD elpasolite
crystals which can be made up from main-group elements (up to bismuth). Our
model's accuracy can be improved systematically, reaching 0.1 eV/atom for a
training set consisting of 10 k crystals. Important bonding trends are
revealed, fluoride is best suited to fit the coordination of the D site which
lowers the formation energy whereas the opposite is found for carbon. The
bonding contribution of elements A and B is very small on average. Low
formation energies result from A and B being late elements from group (II), C
being a late (I) element, and D being fluoride. Out of 2 M crystals, 90 unique
structures are predicted to be on the convex hull---among which NFAlCa,
with peculiar stoichiometry and a negative atomic oxidation state for Al
Crystal Structure Representations for Machine Learning Models of Formation Energies
We introduce and evaluate a set of feature vector representations of crystal
structures for machine learning (ML) models of formation energies of solids. ML
models of atomization energies of organic molecules have been successful using
a Coulomb matrix representation of the molecule. We consider three ways to
generalize such representations to periodic systems: (i) a matrix where each
element is related to the Ewald sum of the electrostatic interaction between
two different atoms in the unit cell repeated over the lattice; (ii) an
extended Coulomb-like matrix that takes into account a number of neighboring
unit cells; and (iii) an Ansatz that mimics the periodicity and the basic
features of the elements in the Ewald sum matrix by using a sine function of
the crystal coordinates of the atoms. The representations are compared for a
Laplacian kernel with Manhattan norm, trained to reproduce formation energies
using a data set of 3938 crystal structures obtained from the Materials
Project. For training sets consisting of 3000 crystals, the generalization
error in predicting formation energies of new structures corresponds to (i)
0.49, (ii) 0.64, and (iii) 0.37 eV/atom for the respective representations
An Ontology for the Materials Design Domain
In the materials design domain, much of the data from materials calculations
are stored in different heterogeneous databases. Materials databases usually
have different data models. Therefore, the users have to face the challenges to
find the data from adequate sources and integrate data from multiple sources.
Ontologies and ontology-based techniques can address such problems as the
formal representation of domain knowledge can make data more available and
interoperable among different systems. In this paper, we introduce the
Materials Design Ontology (MDO), which defines concepts and relations to cover
knowledge in the field of materials design. MDO is designed using domain
knowledge in materials science (especially in solid-state physics), and is
guided by the data from several databases in the materials design field. We
show the application of the MDO to materials data retrieved from well-known
materials databases.Comment: 16 page
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
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