Self-consistent Electronic Structure Calculations Research Articles

Efficient analytical gradients of property-based diabatic states: Geometry optimizations for localized holes.

We present efficient analytical gradients of property-based diabatic states and couplings using a Lagrangian formalism. Unlike previous formulations, the method achieves a computational scaling that is independent of the number of adiabatic states used to construct the diabats. The approach is generalizable to other property-based diabatization schemes and electronic structure methods as long as analytical energy gradients are available and integral derivatives with the property operator can be formed. We also introduce a scheme to phase and reorder diabats to ensure their continuity between molecular configurations. We demonstrate this for the specific case of Boys diabatic states obtained from state-averaged complete active space self-consistent field electronic structure calculations with GPU acceleration in the TeraChem package. The method is used to test the Condon approximation for the hole transfer in an explicitly solvated model DNA oligomer.

Average-atom model with Siegert states.

In plasmas, electronic states can be well-localized bound states or itinerant free states, or something in between. In self-consistent treatments of plasma electronic structure such as the average-atom model, all states must be accurately resolved in order to achieve a converged numerical solution. This is a challenging numerical and algorithmic problem in large part due to the continuum of free states which is relatively expensive and difficult to resolve accurately. Siegert states are an appealing alternative. They form a complete eigenbasis with a purely discrete spectrum while still being equivalent to a representation in terms of the usual bound states and free states. However, many of their properties are unintuitive, and it is not obvious that they are suitable for self-consistent plasma electronic structure calculations. Here it is demonstrated that Siegert states can be used to accurately solve an average-atom model and offer advantages over the traditional finite-difference approach, including a concrete physical picture of pressure ionization and continuum resonances.

Charge self-consistent electronic structure calculations with dynamical mean-field theory using Quantum ESPRESSO, Wannier 90 and TRIQS

We present a fully charge self-consistent implementation of dynamical mean field theory (DMFT) combined with density functional theory (DFT) for electronic structure calculations of materials with strong electronic correlations. The implementation uses the Quantum ESPRESSO package for the DFT calculations, the Wannier90 code for the up-/down-folding and the TRIQS software package for setting up and solving the DMFT equations. All components are available under open source licenses, are MPI-parallelized, fully integrated in the respective packages, and use an hdf5 archive interface to eliminate file parsing. We show benchmarks for three different systems that demonstrate excellent agreement with existing DFT + DMFT implementations in other ab initio electronic structure codes.

The effects of alkaline-earth-metal-element doping on the thermoelectric properties of β-Zn4Sb3

The effects of alkaline-earth metal elements Ca, Sr, and Ba on the electronic structure and thermoelectric properties of β-Zn4Sb3 were investigated by performing self-consistent ab initio electronic structure calculations within density functional theory and solving the Boltzmann transport equations within the relaxation time approximation. The results demonstrate that these alkaline-earth metal elements with s orbitals could introduce giant sharp resonant peaks in the electronic density of states (DOS) near the host valence band maximum in energy. And these deliberately engineered DOS peaks result in a sharp increase of the room-temperature Seebeck coefficient of β-Zn4Sb3 by a factor of nearly 8/9/19, respectively. Additionally, with the simultaneous increase of conductivity and decline of carrier thermal conductivity upon Ca/Sr/Ba doping, potentially, at least, 10/4/2-fold increase in optimizing power factor, and 14/12/8-fold increase in thermoelectric figure of merit of β-Zn4Sb3 at room temperature are achieved. And their corresponding optimal Fermi levels are all located near the host valence band maximum.

Direct coherent switching with decay of mixing for intersystem crossing dynamics of thioformaldehyde: The effect of decoherence

We evaluate the effect of electronic decoherence on intersystem crossing in the photodynamics of thioformaldehyde. First, we show that the state-averaged complete-active-space self-consistent field electronic structure calculations with a properly chosen active space of 12 active electrons in 10 active orbitals can predict the potential energy surfaces and the singlet-triplet spin-orbit couplings quite well for CH2S, and we use this method for direct dynamics by coherent switching with decay of mixing (CSDM). We obtain similar dynamical results with CSDM or by adding energy-based decoherence to trajectory surface hopping, with the population of triplet states tending to a small steady-state value over 500 fs. Without decoherence, the state populations calculated by the conventional trajectory surface hopping method or the semiclassical Ehrenfest method gradually increase. This difference shows that decoherence changes the nature of the results not just quantitatively but qualitatively.

Quasi-Fermi level splitting in nanoscale junctions from ab initio

The splitting of quasi-Fermi levels (QFLs) represents a key concept utilized to describe finite-bias operations of semiconductor devices, but its atomic-scale characterization remains a significant challenge. Herein, the nonequilibrium QFL or electrochemical potential profiles within single-molecule junctions obtained from the first-principles multispace constrained-search density-functional formalism are presented. Benchmarking the standard nonequilibrium Green's function calculation results, it is first established that algorithmically the notion of separate electrode-originated nonlocal QFLs should be maintained within the channel region during self-consistent finite-bias electronic structure calculations. For the insulating hexandithiolate junction, the QFL profiles exhibit discontinuities at the left and right electrode interfaces and across the molecule the accompanying electrostatic potential drops linearly and Landauer residual-resistivity dipoles are uniformly distributed. For the conducting hexatrienedithiolate junction, on the other hand, the electrode QFLs penetrate into the channel region and produce split QFLs. With the highest occupied molecular orbital entering the bias window and becoming a good transport channel, the split QFLs are accompanied by the nonlinear electrostatic potential drop and asymmetric Landauer residual-resistivity dipole formation. Our findings underscore the importance of the first-principles extraction of QFLs in nanoscale junctions and point to a future direction for the computational design of next-generation semiconductor devices.

The effects of transition element doping on the thermoelectric properties of β-Zn4Sb3

The effects of transition elements Fe, Co, and Ni on the electronic structure and thermoelectric properties of β-Zn4Sb3 were investigated by performing self-consistent ab initio electronic structure calculations within density functional theory and solving the Boltzmann transport equations within the relaxation time approximation. The results demonstrate that these transition elements with 3d orbitals could introduce giant sharp resonant peaks in the electronic density of states (DOS) near the host valence band maximum or conduction band minimum in energy. And these deliberately engineered DOS peaks result in a sharp increase of the room-temperature Seebeck coefficient of β-Zn4Sb3 by a factor of nearly 60, 80 and 130, respectively. Additionally, with the simultaneous decline of carrier thermal conductivity upon Co/Ni doping, potentially, at least, 1.21/1.13-fold increase in thermoelectric figure of merit of β-Zn4Sb3 at room temperature are achieved, indicating that the substitution of Co and Ni for Zn can effectively elevate thermoelectric performance of β-Zn4Sb3.

Monopole mining method for high-throughput screening for Weyl semimetals

Although topological invariants have been introduced to classify the appearance of protected electronic states at surfaces of insulators, there are no corresponding indexes for Weyl semimetals whose nodal points may appear randomly in the bulk Brillouin Zone (BZ). Here we use a well-known result that every Weyl point acts as a Dirac monopole and generates integer Berry flux to search for the monopoles on rectangular BZ grids that are commonly employed in self-consistent electronic structure calculations. The method resembles data mining technology of computer science and is demonstrated on locating the Weyl points in known Weyl semimetals. It is subsequently used in high throughput screening several hundreds of compounds and predicting a dozen new materials hosting nodal Weyl points and/or lines.

The Structure of Electronic States and Optical Properties of Cr80Al20 Compound

Ellipsometric studies of the optical properties of the intermetallic Cr80Al20 compound in the spectral range of 0.22–15 μm have been performed. The self-consistent calculation of the electronic structure and density of the electronic states of the material has been carried out. The spectrum of interband light absorption is interpreted on the basis of comparative analysis of theoretical and experimental dispersion dependences of optical conductivity. A number of characteristics of conduction electrons have been determined.

Investigation of the electronic structure of tetragonal B3N3 under pressure

In this paper, we perform self-consistent field relaxation and electronic structure calculations of tetragonal B3N3 based on density functional theory, using LDA pseudopotential in the pressure range between − 30 and + 160 GPa. Although metallic B3N3 has a honeycomb structure, according to the electronic band structure, it has bulk properties (not layered) with effective mass non-interacting electron gas behavior near Fermi level (not Dirac massless) and a small anisotropy, about 0.56 in effective mass in the direction of MA relative to XM. Electronic calculations of the B3N3 under pressure show that increasing positive pressure causes the decrease of Fermi energy and total electronic density of states at Fermi level, due to the ionic bonding nature in the B3N3. The Fermi energy increases a little in pressure ranges of about + 100 to + 160 GPa. According to performed projected density of states calculations of the B3N3 under pressure, which p orbitals of boron and nitrogen atoms with three sp2 hybridized bonding have the most contribution in the electronic states at Fermi level, that have spatial distribution perpendicular to honeycomb planes in pressure range of − 30 to + 160 GPa, like p z orbitals in graphene. In overall, the contribution of the p orbitals of nitrogen atoms is greater than similar p orbitals of boron atoms. Accordingly, the orbitals of nitrogen and boron atoms with higher order, sp3 hybridized bonding have negligible electronic contribution at Fermi level in the all pressure range.

Hybrid-DFT + Vw method for band structure calculation of semiconducting 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 known 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 + Vw method (Ivády et al 2014 Phys. Rev. B 90 035146). Based on the analogy of quasi-particle equations and hybrid-DFT single particle equations, here we demonstrate that parameters of hybrid-DFT + Vw functional can be determined from approximate theoretical quasi-particle spectra without any fitting to experiment. The proposed method is illustrated on the charge self-consistent electronic structure calculation for cerium dioxide where itinerant valence states interact with well-localized 4f atomic like states, making this system challenging for conventional methods, either hybrid-DFT or LDA + U, and therefore allowing for a demonstration of the advantages of the proposed scheme.

Computational Insights into Materials and Interfaces for Capacitive Energy Storage.

Supercapacitors such as electric double‐layer capacitors (EDLCs) and pseudocapacitors are becoming increasingly important in the field of electrical energy storage. Theoretical study of energy storage in EDLCs focuses on solving for the electric double‐layer structure in different electrode geometries and electrolyte components, which can be achieved by molecular simulations such as classical molecular dynamics (MD), classical density functional theory (classical DFT), and Monte‐Carlo (MC) methods. In recent years, combining first‐principles and classical simulations to investigate the carbon‐based EDLCs has shed light on the importance of quantum capacitance in graphene‐like 2D systems. More recently, the development of joint density functional theory (JDFT) enables self‐consistent electronic‐structure calculation for an electrode being solvated by an electrolyte. In contrast with the large amount of theoretical and computational effort on EDLCs, theoretical understanding of pseudocapacitance is very limited. In this review, we first introduce popular modeling methods and then focus on several important aspects of EDLCs including nanoconfinement, quantum capacitance, dielectric screening, and novel 2D electrode design; we also briefly touch upon pseudocapactive mechanism in RuO2. We summarize and conclude with an outlook for the future of materials simulation and design for capacitive energy storage.

Task-based Parallel Computation of the Density Matrix in Quantum-based Molecular Dynamics using Graph Partitioning

Quantum-based molecular dynamics (QMD) is a highly accurate and transferable method for material science simulations. However, the time scales and system sizes accessible to QMD are typically limited to picoseconds and a few hundred atoms. These constraints arise due to expensive self-consistent ground-state electronic structure calculations that can often scale cubically with the number of atoms. Linearly scaling methods depend on computing the density matrix P from the Hamiltonian matrix H by exploiting the sparsity in both matrices. The second-order spectral projection (SP2) algorithm is an O(N) algorithm that computes P with a sequence of 40-50 matrix-matrix multiplications. In this paper, we present task-based implementations of a recently developed data-parallel graph-based approach to the SP2 algorithm, G-SP2. We represent the density matrix P as an undirected graph and use graph partitioning techniques to divide the computation into smaller independent tasks. The partitions thus obtained are generally not of equal size and give rise to undesirable load imbalances in standard MPI-based implementations. This load-balancing challenge can be mitigated by dynamically scheduling parallel computations at runtime using task-based programming models. We develop task-based implementations of the data-parallel G-SP2 algorithm using both Intel's Concurrent Collections (CnC) as well as the Charm++ programming model and evaluate these implementations for future use. Scaling and performance results of our implementations are investigated for representative segments of QMD simulations for solvated protein systems containing more than 10,000 atoms.

Vacancy-induced in-gap states in sodium tungsten bronzes: Density functional investigations

We have performed extensive ab-initio self-consistent electronic-structure calculations on WO3 and NaWO3 with single- and double-oxygen-vacancy defects within the framework of density functional theory. Our calculated density of states reveals that the in-gap states in WO3 and NaWO3 are the consequence of oxygen vacancies in the system. The evolution of the induced states occurs from the unpaired electrons donated by the oxygen vacancy. We found that the energy positions of the in-gap states are sensitive to the oxygen vacancy concentrations. The in-gap states in NaWO3 are formed close to the valence band, which are pushed towards the conduction band with the increase in oxygen vacancies, whereas the states are formed mostly in the mid-gap region in the WO3 system. Our finding can now well explain the discrepancy in experimental band dispersion measurements from ARPES with that of WO3 and NaWO3 band calculations.

Theoretical study of SET operation in carbon nanotube memory cell

We present results of self-consistent electronic structure calculations for an electromechanical memory cell consisting of a carbon nanotube (CNT) fabric between titanium leads to elucidate the mechanism whereby the applied bias works to close the current gaps in the CNT fabric. We demonstrate that the asymmetry in the bias conditions required to achieve the “SET” operation of the cell (changing it from a high resistivity to low resistivity) results from the nature of a voltage drop in a compensated semiconducting material and depends sensitively on the background charge as well as on the position of the layer where the conducting gaps occur. The calculations provide insight into the behavior of the material and suggest possible fabrication strategies to modify the functionality.

Complex centers of hydrogen in tin dioxide

Tin dioxide is a wide band-gap semiconductor and is part of a class of promising transparent conducting oxides. It shows n-type conductivity, even when not intentionally doped, and is usually attributed to intrinsic defects. Theoretically, the unintentional doping with hydrogen, either at interstitials or at O sites, has been proposed to provide the shallow donors for the n-type conductivity of SnO2. Since H is an electrically active impurity present in many growth environments, a deeper theoretical understanding of the hydrogen and H-related complexes in SnO2 is highly welcome. We present here the results of ab initio studies, based on self-consistent electronic structure calculations, based on Perdew, Burke and Ernzerhof plus the on-site Coulomb correction and Heyd–Scuseria–Ernzerhof hybrid functional approaches, for several H-related defect centers in SnO2. Isolated substitutional (HO) and interstitial (Hi) impurities, as well as some complexes related to them, like 2H, HO–H VSn–H, VSn–2H, VO–H2, VO–2H and Sni–H, have been analyzed from structural and electronic properties, formation energy and vibrational frequencies. A comparison of our calculated vibrational frequencies with recent infrared measurements (IR) allowed us to ascribe the observed IR peaks to the H-related centers. This, added to the low formation energy of the VO–H2 center, and nudged-elastic band method-based calculations, is a strong indication for this center to be the source of hidden hydrogen in SnO2.

KKR–CPA study of electronic structure and relative stability of Mg2X ([formula omitted] = Si, Ge, Sn) thermoelectrics containing point defects

Different types of chemical disorder like vacancies, interstitials and anti-site defects are investigated in binary Mg2X (X=Si, Ge or Sn) and pseudo-ternary Mg2Si1-xSnx thermoelectric materials from the charge self-consistent electronic structure calculations, using the Korringa–Kohn–Rostoker method with the coherent potential approximation (KKR–CPA). This approach allows to directly compare computational results for possible types of crystal imperfections (treated as random), since the symmetry of the unit cell is kept unchanged in all considered cases. The influence of the aforementioned point defects on electronic structure and relative crystal stability is investigated from computed densities of states (DOS) and formation energy (Eform) analysis, respectively. It is found that the anti-site disorder in stoichiometric Mg2X compounds is highly unfavourable due to constitution of huge and non-bonding impurity peaks at the Fermi level (EF), the conclusion which remains in line with electronegativity and bonding criteria. On the other hand, the small amount of constituent elements (Si or Mg) located in the interstitial void (within ideal stoichiometry) results in strong modification of electronic states on both sides of the energy gap, yielding in principle either p-type (Mg) or n-type (Si) doping. In off-stoichiometric Mg2-xX1-y, the vacancy defects may lead to either p-type (Mg-site) or n-type (X-site) doping, due to EF shifting into the valence or conduction band edge, respectively. However, the total energy analysis indicates that the site preference of such defects’ onset tends to change with increasing atomic number of X. Si vacancy defects energetically favourable in Mg2-xSi1-y (whatever the considered chemical potential limits) are no more valid in Mg2-xSn1-y, where Mg vacancy defects are expected to be preferred. Mg2-xGe1-y exhibits intermediate behaviours.

Self-consistent field theory based molecular dynamics with linear system-size scaling

We present an improved field-theoretic approach to the grand-canonical potential suitable for linear scaling molecular dynamics simulations using forces from self-consistent electronic structure calculations. It is based on an exact decomposition of the grand canonical potential for independent fermions and does neither rely on the ability to localize the orbitals nor that the Hamilton operator is well-conditioned. Hence, this scheme enables highly accurate all-electron linear scaling calculations even for metallic systems. The inherent energy drift of Born-Oppenheimer molecular dynamics simulations, arising from an incomplete convergence of the self-consistent field cycle, is circumvented by means of a properly modified Langevin equation. The predictive power of the present approach is illustrated using the example of liquid methane under extreme conditions.

Enhanced thermoelectric performance with participation of F-electrons in β-Zn4Sb3

The effects of rare-earth element impurities Ce and Pr on the electronic structure and thermoelectric properties of β-Zn4Sb3 were investigated by performing self-consistent ab initio electronic structure calculations within density functional theory and solving the Boltzmann transport equations within the relaxation time approximation. The results demonstrated that these rare-earth element impurities with f orbitals could introduce giant sharp resonant peaks in the density of states (DOS) near the host valence band maximum in energy. And these deliberately engineered DOS peaks result in a sharp increase of the room-temperature Seebeck coefficient and power factor from those of impurity-free system by a factor of 100 and 22, respectively. Additionally, with the simultaneous declining of carrier thermal conductivity, a potential 5-fold increase at least with Ce doping and more than 3 times increase with Pr doping in the thermoelectric figure of merit of β-Zn4Sb3 at room temperature are achieved. The effective DOS restructuring strategy opens up new opportunities for thermoelectric power generation and waste heat recovery at large scale.

Sparse Projected-Gradient Method As a Linear-Scaling Low-Memory Alternative to Diagonalization in Self-Consistent Field Electronic Structure Calculations.

Large-scale electronic structure calculations usually involve huge nonlinear eigenvalue problems. A method for solving these problems without employing expensive eigenvalue decompositions of the Fock matrix is presented in this work. The sparsity of the input and output matrices is preserved at every iteration, and the memory required by the algorithm scales linearly with the number of atoms of the system. The algorithm is based on a projected gradient iteration applied to the constraint fulfillment problem. The computer time required by the algorithm also scales approximately linearly with the number of atoms (or non-null elements of the matrices), and the algorithm is faster than standard implementations of modern eigenvalue decomposition methods for sparse matrices containing more than 50 000 non-null elements. The new method reproduces the sequence of semiempirical SCF iterations obtained by standard eigenvalue decomposition algorithms to good precision.