231 research outputs found
Hybrid-DFT+V method for accurate band structure of correlated transition metal compounds: the case of cerium dioxide
Hybrid functionals' non-local exchange-correlation potential contains a
derivative discontinuity that improves on standard semi-local density
functional theory (DFT) band gaps. Moreover, by careful parameterization,
hybrid functionals can provide self-interaction reduced description of selected
states. On the other hand, the uniform description of all the electronic states
of a given system is a know drawback of these functionals that causes varying
accuracy in the description of states with different degrees of localization.
This limitation can be remedied by the orbital dependent exact exchange
extension of hybrid functionals; the hybrid-DFT+V method [V. Iv{\'a}dy, et
al., Phys. Rev. B 90, 035146 (2014)]. Based on the analogy of quasi-particle
equations and hybrid-DFT single particle equations, here we demonstrate that
parameters of hybrid-DFT+V functional can be determined from approximate
quasi-particle spectra. The proposed technique leads to a reduction of
self-interaction and provides improved description for both / and /
-electrons of the simulated system. The performance of our charge
self-consistent method is illustrated on the electronic structure calculation
of cerium dioxide where good agreement with both quasi-particle and
experimental spectra is achieved
The origin of the core-level binding energy shifts in nanoclusters
We investigate the shifts of the core-level binding energies in small gold
nanoclusters by using {\it ab initio} density functional theory calculations.
The shift of the 4 states is calculated for magic number nanoclusters in a
wide range of sizes and morphologies. We find a non-monotonous behavior of the
core-level shift in nanoclusters depending on the size. We demonstrate that
there are three main contributions to the Au 4 shifts, which depend
sensitively on the interatomic distances, coordination and quantum confinement.
They are identified and explained by the change of the on-site electrostatic
potential.Comment: 7 pages, 9 figure
Efficient and accurate determination of lattice-vacancy diffusion coefficients via non equilibrium ab initio molecular dynamics
We revisit the color-diffusion algorithm [P. C. Aeberhard et al., Phys. Rev.
Lett. 108, 095901 (2012)] in nonequilibrium ab initio molecular dynamics
(NE-AIMD), and propose a simple efficient approach for the estimation of
monovacancy jump rates in crystalline solids at temperatures well below
melting. Color-diffusion applied to monovacancy migration entails that one
lattice atom (colored-atom) is accelerated toward the neighboring defect-site
by an external constant force F. Considering bcc molybdenum between 1000 and
2800 K as a model system, NE-AIMD results show that the colored-atom jump rate
k_{NE} increases exponentially with the force intensity F, up to F values far
beyond the linear-fitting regime employed previously. Using a simple model, we
derive an analytical expression which reproduces the observed k_{NE}(F)
dependence on F. Equilibrium rates extrapolated by NE-AIMD results are in
excellent agreement with those of unconstrained dynamics. The gain in
computational efficiency achieved with our approach increases rapidly with
decreasing temperatures, and reaches a factor of four orders of magnitude at
the lowest temperature considered in the present study
Temperature dependent effective potential method for accurate free energy calculations of solids
We have developed a thorough and accurate method of determining anharmonic
free energies, the temperature dependent effective potential technique (TDEP).
It is based on \emph{ab initio} molecular dynamics followed by a mapping onto a
model Hamiltonian that describes the lattice dynamics. The formalism and the
numerical aspects of the technique are described in details. A number of
practical examples are given, and results are presented, which confirm the
usefulness of TDEP within \emph{ab initio} and classical molecular dynamics
frameworks. In particular, we examine from first-principles the behavior of
force constants upon the dynamical stabilization of body centered phase of Zr,
and show that they become more localized. We also calculate phase diagram for
He modeled with the Aziz \emph{et al.} potential and obtain results which
are in favorable agreement both with respect to experiment and established
techniques
Finite temperature elastic constants of paramagnetic materials within the disordered local moment picture from ab initio molecular dynamics calculations
We present a theoretical scheme to calculate the elastic constants of
magnetic materials in the high-temperature paramagnetic state. Our approach is
based on a combination of disordered local moments picture and ab initio
molecular dynamics (DLM-MD). Moreover, we investigate a possibility to enhance
the efficiency of the simulations of elastic properties using recently
introduced method: symmetry imposed force constant temperature dependent
effective potential (SIFC-TDEP). We have chosen cubic paramagnetic CrN as a
model system. This is done due to its technological importance and its
demonstrated strong coupling between magnetic and lattice degrees of freedom.
We have studied the temperature dependent single-crystal and polycrystalline
elastic constants of paramagentic CrN up to 1200 K. The obtained results at T=
300 K agree well with the experimental values of polycrystalline elastic
constants as well as Poisson ratio at room temperature. We observe that the
Young's modulus is strongly dependent on temperature, decreasing by ~14% from
T=300 K to 1200 K. In addition we have studied the elastic anisotropy of CrN as
a function of temperature and we observe that CrN becomes substantially more
isotropic as the temperature increases. We demonstrate that the use of Birch
law may lead to substantial errors for calculations of temperature induced
changes of elastic moduli. The proposed methodology can be used for accurate
predictions of mechanical properties of magnetic materials at temperatures
above their magnetic order-disorder phase transition.Comment: 1 table, 3 figure
Ab-initio elastic tensor of cubic TiAlN alloy: the dependence of the elastic constants on the size and shape of the supercell model
In this study we discuss the performance of approximate SQS supercell models
in describing the cubic elastic properties of B1 (rocksalt)
TiAlN alloy by using a symmetry based projection technique. We
show on the example of TiAlN alloy, that this projection
technique can be used to align the differently shaped and sized SQS structures
for a comparison in modeling elasticity. Moreover, we focus to accurately
determine the cubic elastic constants and Zener's type elastic anisotropy of
TiAlN. Our best supercell model, that captures accurately both
the randomness and cubic elastic symmetry, results in GPa,
GPa and GPa with 3% of error and for Zener's
elastic anisotropy with 6% of error. In addition, we establish the general
importance of selecting proper approximate SQS supercells with symmetry
arguments to reliably model elasticity of alloys. In general, we suggest the
calculation of nine elastic tensor elements - , , ,
, , , , and , to evaluate and
analyze the performance of SQS supercells in predicting elasticity of cubic
alloys via projecting out the closest cubic approximate of the elastic tensor.
The here described methodology is general enough to be applied in discussing
elasticity of substitutional alloys with any symmetry and at arbitrary
composition.Comment: Submitted to Physical Review
Elinvar effect in Ti simulated by on-the-fly trained moment tensor potential
A combination of quantum mechanics calculations with machine learning (ML)
techniques can lead to a paradigm shift in our ability to predict materials
properties from first principles. Here we show that on-the-fly training of an
interatomic potential described through moment tensors provides the same
accuracy as state-of-the-art {\it ab inito} molecular dynamics in predicting
high-temperature elastic properties of materials with two orders of magnitude
less computational effort. Using the technique, we investigate high-temperature
bcc phase of titanium and predict very weak, Elinvar, temperature dependence of
its elastic moduli, similar to the behavior of the so-called GUM Ti-based
alloys [T. Sato {\ it et al.}, Science {\bf 300}, 464 (2003)]. Given the fact
that GUM alloys have complex chemical compositions and operate at room
temperature, Elinvar properties of elemental bcc-Ti observed in the wide
temperature interval 1100--1700 K is unique.Comment: 15 pages, 4 figure
Identification of Si-vacancy related room temperature qubits in 4H silicon carbide
Identification of microscopic configuration of point defects acting as
quantum bits is a key step in the advance of quantum information processing and
sensing. Among the numerous candidates, silicon vacancy related centers in
silicon carbide (SiC) have shown remarkable properties owing to their
particular spin-3/2 ground and excited states. Although, these centers were
observed decades ago, still two competing models, the isolated negatively
charged silicon vacancy and the complex of negatively charged silicon vacancy
and neutral carbon vacancy [Phys. Rev. Lett.\ \textbf{115}, 247602 (2015)] are
argued as an origin. By means of high precision first principles calculations
and high resolution electron spin resonance measurements, we here unambiguously
identify the Si-vacancy related qubits in hexagonal SiC as isolated negatively
charged silicon vacancies. Moreover, we identify the Si-vacancy qubit
configurations that provide room temperature optical readout.Comment: 3 figure
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