672 research outputs found
Ab initio study on the effects of transition metal doping of Mg2NiH4
Mg2NiH4 is a promising hydrogen storage material with fast (de)hydrogenation
kinetics. Its hydrogen desorption enthalpy, however, is too large for practical
applications. In this paper we study the effects of transition metal doping by
first-principles density functional theory calculations. We show that the
hydrogen desorption enthalpy can be reduced by ~0.1 eV/H2 if one in eight Ni
atoms is replaced by Cu or Fe. Replacing Ni by Co atoms, however, increases the
hydrogen desorption enthalpy. We study the thermodynamic stability of the
dopants in the hydrogenated and dehydrogenated phases. Doping with Co or Cu
leads to marginally stable compounds, whereas doping with Fe leads to an
unstable compound. The optical response of Mg2NiH4 is also substantially
affected by doping. The optical gap in Mg2NiH4 is ~1.7 eV. Doping with Co, Fe
or Cu leads to impurity bands that reduce the optical gap by up to 0.5 eV.Comment: 8 pages, 4 figure
Quantum Size Effects in the Atomistic Structure of Armchair-Nanoribbons
Quantum size effects in armchair graphene nano-ribbons (AGNR) with hydrogen
termination are investigated via density functional theory (DFT) in Kohn-Sham
formulation. "Selection rules" will be formulated, that allow to extract
(approximately) the electronic structure of the AGNR bands starting from the
four graphene dispersion sheets. In analogy with the case of carbon nanotubes,
a threefold periodicity of the excitation gap with the ribbon width (N, number
of carbon atoms per carbon slice) is predicted that is confirmed by ab initio
results. While traditionally such a periodicity would be observed in electronic
response experiments, the DFT analysis presented here shows that it can also be
seen in the ribbon geometry: the length of a ribbon with L slices approaches
the limiting value for a very large width 1 << N (keeping the aspect ratio
small N << L) with 1/N-oscillations that display the electronic selection
rules. The oscillation amplitude is so strong, that the asymptotic behavior is
non-monotonous, i.e., wider ribbons exhibit a stronger elongation than more
narrow ones.Comment: 5 pages, 6 figure
Automation methodologies and large-scale validation for , towards high-throughput calculations
The search for new materials, based on computational screening, relies on
methods that accurately predict, in an automatic manner, total energy,
atomic-scale geometries, and other fundamental characteristics of materials.
Many technologically important material properties directly stem from the
electronic structure of a material, but the usual workhorse for total energies,
namely density-functional theory, is plagued by fundamental shortcomings and
errors from approximate exchange-correlation functionals in its prediction of
the electronic structure. At variance, the method is currently the
state-of-the-art {\em ab initio} approach for accurate electronic structure. It
is mostly used to perturbatively correct density-functional theory results, but
is however computationally demanding and also requires expert knowledge to give
accurate results. Accordingly, it is not presently used in high-throughput
screening: fully automatized algorithms for setting up the calculations and
determining convergence are lacking. In this work we develop such a method and,
as a first application, use it to validate the accuracy of using the
PBE starting point, and the Godby-Needs plasmon pole model
(@PBE), on a set of about 80 solids. The results of the
automatic convergence study utilized provides valuable insights. Indeed, we
find correlations between computational parameters that can be used to further
improve the automatization of calculations. Moreover, we find that
@PBE shows a correlation between the PBE and the
@PBE gaps that is much stronger than that between and
experimental gaps. However, the @PBE gaps still describe
the experimental gaps more accurately than a linear model based on the PBE
gaps.Comment: 12 pages, 11 figure
The PseudoDojo: Training and grading a 85 element optimized norm-conserving pseudopotential table
First-principles calculations in crystalline structures are often performed
with a planewave basis set. To make the number of basis functions tractable two
approximations are usually introduced: core electrons are frozen and the
diverging Coulomb potential near the nucleus is replaced by a smoother
expression. The norm-conserving pseudopotential was the first successful method
to apply these approximations in a fully ab initio way. Later on, more
efficient and more exact approaches were developed based on the ultrasoft and
the projector augmented wave formalisms. These formalisms are however more
complex and developing new features in these frameworks is usually more
difficult than in the norm-conserving framework. Most of the existing tables of
norm- conserving pseudopotentials, generated long ago, do not include the
latest developments, are not systematically tested or are not designed
primarily for high accuracy. In this paper, we present our PseudoDojo framework
for developing and testing full tables of pseudopotentials, and demonstrate it
with a new table generated with the ONCVPSP approach. The PseudoDojo is an open
source project, building on the AbiPy package, for developing and
systematically testing pseudopotentials. At present it contains 7 different
batteries of tests executed with ABINIT, which are performed as a function of
the energy cutoff. The results of these tests are then used to provide hints
for the energy cutoff for actual production calculations. Our final set
contains 141 pseudopotentials split into a standard and a stringent accuracy
table. In total around 70.000 calculations were performed to test the
pseudopotentials. The process of developing the final table led to new insights
into the effects of both the core-valence partitioning and the non-linear core
corrections on the stability, convergence, and transferability of
norm-conserving pseudopotentials. ...Comment: abstract truncated, 17 pages, 25 figures, 8 table
A model for the formation energies of alanates and boranates
We develop a simple model for the formation energies (FEs) of alkali and
lkaline earth alanates and boranates, based upon ionic bonding between metal
cations and (AlH4)- or (BH4)- anions. The FEs agree well with values obtained
from first principles calculations and with experimental FEs. The model shows
that details of the crystal structure are relatively unimportant. The small
size of the (BH4)- anion causes a strong bonding in the crystal, which makes
boranates more stable than alanates. Smaller alkali or alkaline earth cations
do not give an increased FE. They involve a larger ionization potential that
compensates for the increased crystal bonding.Comment: 3 pages, 2 figure
Effect of tissue-grouped regulatory variants associated to type 2 diabetes in related secondary outcomes
Genome-wide association studies have identified over five hundred loci that contribute to variation in type 2 diabetes (T2D), an established risk factor for many diseases. However, the mechanisms and extent through which these loci contribute to subsequent outcomes remain elusive. We hypothesized that combinations of T2D-associated variants acting on tissue-specific regulatory elements might account for greater risk for tissue-specific outcomes, leading to diversity in T2D disease progression. We searched for T2D-associated variants acting on regulatory elements and expression quantitative trait loci (eQTLs) in nine tissues. We used T2D tissue-grouped variant sets as genetic instruments to conduct 2-Sample Mendelian Randomization (MR) in ten related outcomes whose risk is increased by T2D using the FinnGen cohort. We performed PheWAS analysis to investigate whether the T2D tissue-grouped variant sets had specific predicted disease signatures. We identified an average of 176 variants acting in nine tissues implicated in T2D, and an average of 30 variants acting on regulatory elements that are unique to the nine tissues of interest. In 2-Sample MR analyses, all subsets of regulatory variants acting in different tissues were associated with increased risk of the ten secondary outcomes studied on similar levels. No tissue-grouped variant set was associated with an outcome significantly more than other tissue-grouped variant sets. We did not identify different disease progression profiles based on tissue-specific regulatory and transcriptome information. Bigger sample sizes and other layers of regulatory information in critical tissues may help identify subsets of T2D variants that are implicated in certain secondary outcomes, uncovering system-specific disease progression
First-principles study of the optical properties of MgxTi(1-x)H2
The optical and electronic properties of Mg-Ti hydrides are studied using
first-principles density functional theory. Dielectric functions are calculated
for MgxTi(1-x)H2 with compositions x = 0.5, 0.75, and 0.875. The structure is
that of fluorite TiH2 where both Mg and Ti atoms reside at the Ti positions of
the lattice. In order to assess the effect of randomness in the Mg and Ti
occupations we consider both highly ordered structures, modeled with simple
unit cells of minimal size, and models of random alloys. These are simulated by
super cells containing up to 64 formula units (Z = 64). All compositions and
structural models turn out metallic, hence the dielectric functions contain
interband and intraband free electron contributions. The former are calculated
in the independent particle random phase approximation. The latter are modeled
based upon the intraband plasma frequencies, which are also calculated from
first-principles. Only for the models of the random alloys we obtain a black
state, i.e. low reflection and transmission in the energy range from 1 to 6 eV.Comment: 7 pages, 8 figure
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