8 research outputs found
Electronic Structure of Cesium-based Photocathode Materials from Density Functional Theory: Performance of PBE, SCAN, and HSE06 functionals
The development of novel materials for vacuum electron sources in particle
accelerators is an active field of research that can greatly benefit from the
results of \textit{ab initio} calculations for the characterization of the
electronic structure of target systems. As state-of-the-art many-body
perturbation theory calculations are too expensive for large-scale material
screening, density functional theory offers the best compromise between
accuracy and computational feasibility. The quality of the obtained results,
however, crucially depends on the choice of the exchange-correlation potential,
. To address this essential point, we systematically analyze the
performance of three popular approximations of (PBE, SCAN, and HSE06)
on the structural and electronic properties of bulk CsSb and CsTe, two
representative materials of Cs-based semiconductors employed in photocathode
applications. Among the adopted approximations, PBE shows expectedly the
largest discrepancies from the target: the unit cell volume is overestimated
compared to the experimental value, while the band gap is severely
underestimated. On the other hand, both SCAN and HSE06 perform remarkably well
in reproducing both structural and electronic properties. Spin-orbit coupling,
which mainly impacts the valence region of both materials inducing a band
splitting and, consequently, a band-gap reduction of the order of 0.2 eV, is
equally captured by all functionals. Our results indicate SCAN as the best
trade-off between accuracy and computational costs, outperforming the
considerably more expensive HSE06
Speeding-up Hybrid Functional based Ab Initio Molecular Dynamics using Multiple Time-stepping and Resonance Free Thermostat
Ab initio molecular dynamics (AIMD) based on density functional theory (DFT)
has become a workhorse for studying the structure, dynamics, and reactions in
condensed matter systems. Currently, AIMD simulations are primarily carried out
at the level of generalized gradient approximation (GGA), which is at the 2nd
rung of DFT-functionals in terms of accuracy. Hybrid DFT functionals which form
the 4th rung in the accuracy ladder, are not commonly used in AIMD simulations
as the computational cost involved is times or higher. To facilitate AIMD
simulations with hybrid functionals, we propose here an approach that could
speed up the calculations by ~30 times or more for systems with a few hundred
of atoms. We demonstrate that, by achieving this significant speed up and
making the compute time of hybrid functional based AIMD simulations at par with
that of GGA functionals, we are able to study several complex condensed matter
systems and model chemical reactions in solution with hybrid functionals that
were earlier unthinkable to be performed.Comment: 45 pages, 9 figures, 5 table
Accuracy of dielectric-dependent hybrid functionals in the prediction of optoelectronic properties of metal oxide semiconductors: a comprehensive comparison with many-body GW and experiments
Understanding the electronic structure of metal oxide semiconductors is crucial to their numerous technological applications, such as photoelectrochemical water splitting and solar cells. The needed experimental and theoretical knowledge goes beyond that of pristine bulk crystals, and must include the effects of surfaces and interfaces, as well as those due to the presence of intrinsic defects (e.g. oxygen vacancies), or dopants for band engineering. In this review, we present an account of the recent efforts in predicting and understanding the optoelectronic properties of oxides using ab initio theoretical methods. In particular, we discuss the performance of recently developed dielectric-dependent hybrid functionals, providing a comparison against the results of many-body GW calculations, including G 0 W 0 as well as more refined approaches, such as quasiparticle self-consistent GW. We summarize results in the recent literature for the band gap, the band level alignment at surfaces, and optical transition energies in defective oxides, including wide gap oxide semiconductors and transition metal oxides. Correlated transition metal oxides are also discussed. For each method, we describe successes and drawbacks, emphasizing the challenges faced by the development of improved theoretical approaches. The theoretical section is preceded by a critical overview of the main experimental techniques needed to characterize the optoelectronic properties of semiconductors, including absorption and reflection spectroscopy, photoemission, and scanning tunneling spectroscopy (STS)
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Improved treatment of exact exchange in Quantum ESPRESSO
We present an algorithm and implementation for the parallel computation of exact exchange in Quantum ESPRESSO (QE) that exhibits greatly improved strong scaling. QE is an open-source software package for electronic structure calculations using plane wave density functional theory, and supports the use of local, semi-local, and hybrid DFT functionals. Wider application of hybrid functionals is desirable for the improved simulation of electronic band energy alignments and thermodynamic properties, but the computational complexity of evaluating the exact exchange potential limits the practical application of hybrid functionals to large systems and requires efficient implementations. We demonstrate that existing implementations of hybrid DFT that utilize a single data structure for both the local and exact exchange regions of the code are significantly limited in the degree of parallelization achievable. We present a band-pair parallelization approach, in which the calculation of exact exchange is parallelized and evaluated independently from the parallelization of the remainder of the calculation, with the wavefunction data being efficiently transformed on-the-fly into a form that is optimal for each part of the calculation. For a 64 water molecule supercell, our new algorithm reduces the overall time to solution by nearly an order of magnitude
Improved treatment of exact exchange in Quantum ESPRESSO
© 2017 We present an algorithm and implementation for the parallel computation of exact exchange in Quantum ESPRESSO (QE) that exhibits greatly improved strong scaling. QE is an open-source software package for electronic structure calculations using plane wave density functional theory, and supports the use of local, semi-local, and hybrid DFT functionals. Wider application of hybrid functionals is desirable for the improved simulation of electronic band energy alignments and thermodynamic properties, but the computational complexity of evaluating the exact exchange potential limits the practical application of hybrid functionals to large systems and requires efficient implementations. We demonstrate that existing implementations of hybrid DFT that utilize a single data structure for both the local and exact exchange regions of the code are significantly limited in the degree of parallelization achievable. We present a band-pair parallelization approach, in which the calculation of exact exchange is parallelized and evaluated independently from the parallelization of the remainder of the calculation, with the wavefunction data being efficiently transformed on-the-fly into a form that is optimal for each part of the calculation. For a 64 water molecule supercell, our new algorithm reduces the overall time to solution by nearly an order of magnitude