1,721,089 research outputs found
Large scale GW calculations
Full text linkWe present GW calculations of molecules, ordered and disordered solids and interfaces, which employ an efficient contour deformation technique for frequency integration and do not require the explicit evaluation of virtual electronic states nor the inversion of dielectric matrices. We also present a parallel implementation of the algorithm which takes advantage of separable expressions of both the single particle Green's function and the screened Coulomb interaction. The method can be used starting from density functional theory calculations performed with semilocal or hybrid functionals. We applied the newly developed technique to GW calculations of systems of unprecedented size, including water/semiconductor interfaces with thousands of electrons
Design of defect spins in piezoelectric aluminum nitride for solid-state hybrid quantum technologies
Full text link: Spin defects in wide-band gap semiconductors are promising systems for the realization of quantum bits, or qubits, in solid-state environments. To date, defect qubits have only been realized in materials with strong covalent bonds. Here, we introduce a strain-driven scheme to rationally design defect spins in functional ionic crystals, which may operate as potential qubits. In particular, using a combination of state-of-the-art ab-initio calculations based on hybrid density functional and many-body perturbation theory, we predicted that the negatively charged nitrogen vacancy center in piezoelectric aluminum nitride exhibits spin-triplet ground states under realistic uni- and bi-axial strain conditions; such states may be harnessed for the realization of qubits. The strain-driven strategy adopted here can be readily extended to a wide range of point defects in other wide-band gap semiconductors, paving the way to controlling the spin properties of defects in ionic systems for potential spintronic technologies
Vibrationally resolved optical excitations of the nitrogen-vacancy center in diamond
Full text linkA comprehensive description of the optical cycle of spin defects in solids requires the understanding of the electronic and atomistic structure of states with different spin multiplicity, including singlet states which are particularly challenging from a theoretical standpoint. We present a general framework, based on spin-flip time-dependent density function theory, to determine the excited state potential energy surfaces of the many-body singlet states of spin defects; we then predict the vibrationally resolved absorption spectrum between singlet shelving states of a prototypical defect, the nitrogen-vacancy center in diamond. Our results, which are in very good agreement with experiments, provide an interpretation of the measured spectra and reveal the key role of specific phonons in determining absorption processes, and the notable influence of non-adiabatic interactions. The insights gained from our calculations may be useful in defining strategies to improve infrared-absorption-based magnetometry and optical pumping schemes. The theoretical framework developed here is general and applicable to a variety of other spin defects and materials
Nonempirical Range-Separated Hybrid Functional with Spatially Dependent Screened Exchange
Full text linkElectronic structure calculations based on density functionaltheory(DFT) have successfully predicted numerous ground-state propertiesof a variety of molecules and materials. However, exchange and correlationfunctionals currently used in the literature, including semilocaland hybrid functionals, are often inaccurate to describe the electronicproperties of heterogeneous solids, especially systems composed ofbuilding blocks with large dielectric mismatch. Here, we present adielectric-dependent range-separated hybrid functional, screened-exchangerange-separated hybrid (SE-RSH), for the investigation of heterogeneousmaterials. We define a spatially dependent fraction of exact exchangeinspired by the static Coulomb-hole and screened-exchange (COHSEX)approximation used in many-body perturbation theory, and we show thatthe proposed functional accurately predicts the electronic structureof several nonmetallic interfaces, three- and two-dimensional, pristine,and defective solids and nanoparticles
First-principles calculations of electronic coupling effects in silicon nanocrystals: Influence on near band-edge states and on carrier multiplication processes
No full textArrays of closely packed nanocrystals show interesting properties that can be exploited to induce new features in nanostructured optoelectronic devices. In this work we study, by first principles calculations, effects induced on near band-edge states and on carrier multiplication by nanocrystals interplay. By considering both hydrogenated and oxygenated structures, we prove that interaction between silicon nanocrystals can alter both the energy gap of the system and dynamics of excited states with a relevance that depends on the nanocrystal-nanocrystal separation, on nanocrystals orientation and on nanocrystals surface properties
Machine learning dielectric screening for the simulation of excited state properties of molecules and materials
No full text: Accurate and efficient calculations of absorption spectra of molecules and materials are essential for the understanding and rational design of broad classes of systems. Solving the Bethe-Salpeter equation (BSE) for electron-hole pairs usually yields accurate predictions of absorption spectra, but it is computationally expensive, especially if thermal averages of spectra computed for multiple configurations are required. We present a method based on machine learning to evaluate a key quantity entering the definition of absorption spectra: the dielectric screening. We show that our approach yields a model for the screening that is transferable between multiple configurations sampled during first principles molecular dynamics simulations; hence it leads to a substantial improvement in the efficiency of calculations of finite temperature spectra. We obtained computational gains of one to two orders of magnitude for systems with 50 to 500 atoms, including liquids, solids, nanostructures, and solid/liquid interfaces. Importantly, the models of dielectric screening derived here may be used not only in the solution of the BSE but also in developing functionals for time-dependent density functional theory (TDDFT) calculations of homogeneous and heterogeneous systems. Overall, our work provides a strategy to combine machine learning with electronic structure calculations to accelerate first principles simulations of excited-state properties
GW100: Comparison of Methods and Accuracy of Results Obtained with the WEST Code
No full textThe
reproducibility of calculations carried out within many-body
perturbation theory at the G0W0 level is assessed for 100 closed shell molecules and
compared to that of density functional theory. We consider vertical
ionization potentials (VIP) and electron affinities (VEA) obtained
with five different codes: BerkeleyGW, FHI-aims, TURBOMOLE, VASP,
and WEST. We review the approximations and parameters that control
the accuracy of G0W0 results in each code, and we discuss in detail the effect
of extrapolation techniques for the parameters entering the WEST code.
Differences between the VIP and VEA computed with the various codes
are within ∼60 and ∼120 meV, respectively, which is
up to four times larger than in the case of the best results obtained
with DFT codes. Vertical ionization potentials are validated against
experiment and CCSD(T) quantum chemistry results showing a mean absolute
relative error of ∼4% for data obtained with WEST. Our analysis
of the differences between localized orbitals and plane-wave implementations
points out molecules containing Cu, I, Ga, and Xe as major sources
of discrepancies, which call for a re-evaluation of the pseudopotentials
used for these systems in G0W0 calculations
Quantum Embedding Theory for Strongly Correlated States in Materials
No full text: Quantum embedding theories are promising approaches to investigate strongly correlated electronic states of active regions of large-scale molecular or condensed systems. Notable examples are spin defects in semiconductors and insulators. We present a detailed derivation of a quantum embedding theory recently introduced, which is based on the definition of effective Hamiltonians. The effect of the environment on a chosen active space is accounted for through screened Coulomb interactions evaluated using density functional theory. Importantly, the random phase approximation is not required, and the evaluation of virtual electronic orbitals is circumvented with algorithms previously developed in the context of calculations based on many-body perturbation theory. In addition, we generalize the quantum embedding theory to active spaces composed of orbitals that are not eigenstates of Kohn-Sham Hamiltonians. Finally, we report results for spin defects in semiconductors
Quantum embedding theories to simulate condensed systems on quantum computers
Full text linkQuantum computers hold promise to improve the efficiency of quantum simulations of materials and to enable the investigation of systems and properties that are more complex than tractable at present on classical architectures. Here, we discuss computational frameworks to carry out electronic structure calculations of solids on noisy intermediate-scale quantum computers using embedding theories, and we give examples for a specific class of materials, that is, solid materials hosting spin defects. These are promising systems to build future quantum technologies, such as quantum computers, quantum sensors and quantum communication devices. Although quantum simulations on quantum architectures are in their infancy, promising results for realistic systems appear to be within reach
The role of defects and excess surface charges at finite temperature for optimizing oxide photoabsorbers
No full text: Computational screening of materials for solar to fuel conversion technologies has mostly focused on bulk properties, thus neglecting the structure and chemistry of surfaces and interfaces with water. We report a finite temperature study of WO3, a promising anode for photoelectrochemical cells, carried out using first-principles molecular dynamics simulations coupled with many-body perturbation theory. We identified three major factors determining the chemical reactivity of the material interfaced with water: the presence of surface defects, the dynamics of excess charge at the surface, and finite temperature fluctuations of the surface electronic orbitals. These general descriptors are essential for the understanding and prediction of optimal oxide photoabsorbers for water oxidation
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