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Searching for high-activity, stable and low-cost catalysts toward oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) are of significant importance to the development of renewable energy technologies. By using the computational screening method based on the density functional theory (DFT), we have systematically studied a wide range of transition metal (TM) atoms doped a defective BC3 monolayer (B atom vacancy VB and C atom vacancy VC), denoted as TM@VB and TM@VC (TM = Mn, Fe, Co, Ni, Cu, Ru, Rh, Pd, Ir and Pt), as efficient single atom catalysts for OER and ORR. The calculated results show that all the considered TM atoms can tightly bind with the defective BC3 monolayers to prevent the atomically dispersed atoms from clustering. The interaction strength between intermediates (HO*, O* and HOO*) and catalyst govern the catalytic activities of OER and ORR, which has a direct correlation with the d-band center (ed) of the TM active site that can be tuned by adjusting TM atoms with various d electron numbers. For TM@VB catalysts, it was found that the best catalyst for OER is Co@VB with an overpotential ηOER of 0.43 V, followed by Rh@VB (ηOER = 0.49 V), while for ORR, Rh@VB exhibits the lowest overpotential ηORR of 0.40 V, followed by Pd@VB (ηORR = 0.45 V). For TM@VC catalysts, the best catalyst for OER is Ni@VC (ηOER = 0.47 V), followed by Pt@VC (ηOER = 0.53 V), and for ORR, Pd@VC exhibits the highest activity with ηORR of 0.45 V. The results suggest that the high activity of the newly predicted well dispersed Rh@VB SAC is comparable to that of noble metal oxide benchmark catalysts for both OER and ORR. Importantly, Rh@VB may remain stable against dissolution at pH = 0 condition. The high energy barrier prevents the isolated Rh atom from clustering and ab initio molecule dynamic simulation (AIMD) result suggests that Rh@VB can remain stable under 300 K, indicating its kinetic stability. Our findings highlight a novel family of efficient and stable SAC based on carbon material, which offer a useful guideline to screen the metal active site for catalyst designation.

The hot carrier cooling occurs in most photoexcitation-induced phase transitions (PIPTs), but its role has often been neglected in many theoretical simulations as well as in proposed mechanisms. Here, by including the previously ignored hot carrier cooling in real-time time-dependent density functional theory (rt-TDDFT) simulations, we investigated the role of hot carrier cooling in PIPTs. Taking IrTe2 as an example, we reveal that the cooling of hot electrons from the higher energy levels of spatially extended states to the lower energy levels of the localized Ir-Ir dimer antibonding states strengthens remarkably the atomic driving forces and enhances atomic kinetic energy. These two factors combine to dissolute the Ir-Ir dimers on a timescale near the limit of atomic motions, thus initiating a deterministic kinetic phase transition. We further demonstrate that the subsequent cooling induces nonradiative recombination of photoexcited electrons and holes, leading to the ultrafast recovery of the Ir-Ir dimers observed experimentally. These findings provide a complete picture of the atomic dynamics in optically excited structural phase transitions., Comment: 20 pages, 8 figures

Solid-state Li–S batteries are attractive due to their high energy density and safety. However, it is unclear whether the concepts from liquid electrolytes are applicable in the solid state to improve battery performance. Here, we demonstrate that the nanoscale encapsulation concept based on Li2S@TiS2 core–shell particles, originally developed in liquid electrolytes, is effective in solid polymer electrolytes. Using in situ optical cell and sulfur K-edge X-ray absorption, we find that polysulfides form and are well-trapped inside individual particles by the nanoscale TiS2 encapsulation. This TiS2 encapsulation layer also functions to catalyze the oxidation reaction of Li2S to sulfur, even in solid-state electrolytes, proven by both experiments and density functional theory calculations. A high cell-level specific energy of 427 W·h·kg–1 is achieved by integrating the Li2S@TiS2 cathode with a poly­(ethylene oxide)-based electrolyte and a lithium metal anode. This study points to the fruitful direction of borrowing concepts from liquid electrolytes into solid-state batteries.

A method using machine learning (ML) is proposed to describe metal growth for simulations, which retains the accuracy of ab initio density functional theory (DFT) and results in a thousands-fold reduction in the computational time. This method is based on atomic energy decomposition from DFT calculations. Compared with other ML methods, our energy decomposition approach can yield much more information with the same DFT calculations. This approach is employed for the amorphous sodium system, where only 1000 DFT molecular dynamics images are enough for training an accurate model. The DFT and neural network potential (NNP) are compared for the dynamics to show that similar structural properties are generated. Finally, metal growth experiments from liquid to solid in a small and larger system are carried out to demonstrate the ability of using NNP to simulate the real growth process.

A crystalline solid electrolyte interphase Li2CO3 material with a large band gap shows promise toward next-generation all-solid-state lithium batteries (ASSLBs). However, the inferior ionic diffusivity restricts such structures to a real battery setup. Herein, based on density functional theory calculation and Python materials genomics, we theoretically develop the chemistry and local structural motifs to build a mixed boron-carbon framework Li2+xC1-xBxO3 (LCBO). We examine the electrochemical and chemical stabilities of LCBO-electrode interfaces by analyzing the thermodynamics of formation of interfacial phases. Interestingly, the LCBO material is automatically protected from further decomposition through the self-generated resistive interphase (Li2CO3 and Li3BO3), which gives a wide range of operating potential. LCBO shows high interfacial stability with LiCoO2, LiMnO2, and LiMn2O4. More importantly, the theoretical Li-ion migration barrier of LCBO (x = 0.375) is approximately 0.23 ± 0.02 eV through a cooperative migration mechanism. Therefore, the LCBO material combines high Li-ion diffusivity with good interfacial stability, which makes it a promising solid-state electrolyte material for ASSLBs.

Li1+xAlxTi2−x(PO4)3 (LATP) is attracting attention as a promising inorganic solid electrolyte (ISE) with potential use in all-solid-state lithium-ion batteries. The objective of this paper is to examine and understand the effect of the Al-dopant concentration on the Li-ion diffusion of LATP using density functional theory and the molecular dynamics method. By comparing Li1.16Al0.16Ti1.84(PO4)3 (LATP-0.16) and Li1.33Al0.33Ti1.67(PO4)3 (LATP-0.33) with Li1.5Al0.5Ti1.5(PO4)3 (LATP-0.50), LATP-0.50 is expected to have higher ionic conductivity. The trapping effect of Al-dopants on Li-ions is greatly reduced in LATP-0.50 due to the delocalization of polarization interactions and the depopulation of oxygen atoms, which results in a smooth energy landscape and destabilization of Li-ions. The energy difference of adjacent Li-ions and binding interaction of Li–Li due to a specific local configuration of two Li-ions alternately enable cooperative migration of Li-ions. This understanding of high Li-ion diffusion is important in interpreting the experimental results aiming to assess the effects of Al-dopants on Li-ion conductivity and can be used by researchers to engineer these materials for batteries.

Cross-coupling reactions have developed into powerful approaches for carbon–carbon bond formation. In this work, a Ni-catalyzed migratory Suzuki–Miyaura cross-coupling featuring high benzylic or allylic selectivity has been developed. With this method, unactivated alkyl electrophiles and aryl or vinyl boronic acids can be efficiently transferred to diarylalkane or allylbenzene derivatives under mild conditions. Importantly, unactivated alkyl chlorides can also be successfully used as the coupling partners. To demonstrate the applicability of this method, we showcase that this strategy can serve as a platform for the synthesis of terminal, partially deuterium-labeled molecules from readily accessible starting materials. Experimental studies suggest that migratory cross-coupling products are generated from Ni(0/II) catalytic cycle. Theoretical calculations indicate that the chain-walking occurs at a neutral nickel complex rather than a cationic one. In addition, the original-site cross-coupling products can be obtained by alternating the ligand, wherein the formation of the products has been rationalized by a radical chain process., Migratory cross-coupling reactions are powerful tools to form bonds at predictable positions. Here the authors report a nickel-catalyzed migratory Suzuki–Miyaura cross-coupling of unactivated alkyl electrophiles with aryl and vinyl boron reagents and provide experimental and computational mechanistic evidence.

Ultrafast control of magnetic order by light provides a promising realization for spintronic devices beyond Moore’s Law and has stimulated intense research interest in recent years. Yet, despite 2 decades of debates, the key question of how the spin angular momentum flows on the femtosecond timescale remains open. The lack of direct first-principle methods and pictures for such process exacerbates the issue. Here, we unravel the laser-induced demagnetization mechanism of ferromagnetic semiconductor GaMnAs, using an efficient time-dependent density functional theory approach that enables the direct real-time snapshot of the demagnetization process. Our results show a clear spin-transfer trajectory from the localized Mn- [Formula: see text] electrons to itinerant carriers within 20 fs, illustrating the dominant role of [Formula: see text] interaction. We find that the total spin of localized electrons and itinerant carriers is not conserved in the presence of spin-orbit coupling (SOC). Immediately after laser excitation, a growing percentage of spin-angular momentum is quickly transferred to the electron orbital via SOC in about 1 ps, then slowly to the lattice via electron–phonon coupling in a few picoseconds, responsible for the 2-stage process observed experimentally. The spin-relaxation time via SOC is about 300 fs for itinerant carriers and about 700 fs for Mn- [Formula: see text] electrons. These results provide a quantum-mechanical microscopic picture for the long-standing questions regarding the channels and timescales of spin transfer, as well as the roles of different interactions underlying the GaMnAs demagnetization process.

Molecule damage under TEM electron beam illumination is studied using a systematical ab initio method. Three main dissociation paths are revealed which explains the experimentally observed mass spectra of the dissociation fragments of the C2H6O2+., Radiolysis ionization under electron beam illumination induces dissociation and damage of organic and biological molecules; thus, it is impossible to image the related materials by transmission electron microscopy (TEM). To understand the atomistic mechanism of radiolysis damage, we developed a systematical procedure based on real-time time-dependent density functional theory (rt-TDDFT) for simulating the radiolysis damage processes of molecules; this procedure can describe the ionization cross sections of the electronic states and the fast dissociation processes caused by hot carrier cooling and the Auger decay on deep levels. For the radiolysis damage of C2H6O2, our simulation unexpectedly showed that there is strong competition among three different dissociation paths, including fast dissociation caused by nonadiabatic cooling of the hot carrier; fast dissociation caused by Auger decay, which induces double ionization and Coulomb explosion; and slow dissociation caused by increased kinetic energy. As the energy of the incident electron beam changes, the time scales of these dissociation paths and their relative contributions to the molecule damage change significantly. These simulation results explain the measured mass spectra of the C2H6O2 dissociation fragments and also provide clear competition mechanisms for blocking these dissociation paths in the TEM imaging of organic and biological materials.

Carrier induced defect creation at the semiconductor-oxide interface has been known as the origin of electronic device degradation for a long time, but how exactly the interface lattice can be damaged by carriers (especially low-energy ones) remains unclear. Here we carry out real-time time-dependent density functional theory simulations on concrete $\mathrm{Si}/\mathrm{Si}{\mathrm{O}}_{2}$ interfaces to study the interaction between excited electrons and interface bonds. We show that the normal interface Si-H bonds are generally resistant to electrons due to the delocalized nature and high energy level of the Si-H antibonding states, and due to the high-energy barrier to break the Si-H bond. However, if an additional hydrogen atom exists by attaching to a nearby oxygen atom (forming a ``Si-H\ifmmode\cdot\else\textperiodcentered\fi{}\ifmmode\cdot\else\textperiodcentered\fi{}\ifmmode\cdot\else\textperiodcentered\fi{}H-O'' complex), the Si-H bond will be greatly weakened, including the reduction of energy barrier for bond breaking, and the lowering of the antibonding state energy level which favors electron injection. Together with the multiple vibrational excitation process, the corresponding Si-H bond can be broken much more easily. Thus we propose that the Si-H\ifmmode\cdot\else\textperiodcentered\fi{}\ifmmode\cdot\else\textperiodcentered\fi{}\ifmmode\cdot\else\textperiodcentered\fi{}H-O complex will be the center for defect creation and device degradation. We also explain why such a center might be relatively easy to form during the hydrogen annealing process.

Author(s): Song, Z; Sun, X; Wang, LW | Abstract: Twisted bilayers of van der Waals materials have recently attracted great attention due to their tunable, strongly correlated phenomena. Here, we investigate the chirality-specific physics in three-dimensional moire superlattices induced by Eshelby twist. Our direct density functional theory calculations reveal helical rotation leads to optical circular dichroism and chirality-specific nonlinear Hall effect, even though there is no magnetization or magnetic field. Both these phenomena can be reversed by changing the structural chirality. This provides a path to constructing chirality-specific materials.

Oriented attachment of nanocrystals is an important route to constructing epitaxially-connected nanocrystal superlattices for various applications. During oriented attachment of semiconductor nanocrystals, neck can be formed between nanocrystals and it strongly influences the properties of the resulting superlattice. However, the neck formation mechanism is poorly understood. Here, we use in situ liquid cell transmission electron microscopy to directly observe the initiation and growth of homoepitaxial necks between PbSe nanocrystals with atomic details. We find that neck initiation occurs slowly (~ 10 s) when two nanocrystals approach to each other within an edge-to-edge distance of 0.6 nm. During neck initiation, Pb and Se atoms defuse from other facets into the gap, forming “dynamic reversible” filaments. Once the filament (neck) width is larger than a critical size of 0.9 nm, it gradually (15 s) widens into a 3-nm-wide neck. The atomic structure of the neck is further obtained using ex situ aberration-corrected scanning TEM imaging. Neck initiation and growth mechanisms are elucidated with density functional theory calculations. Our direct unveiling of the atomic pathways of neck formation during oriented attachment shed light into the fabrication of nanocrystal superlattices with improved structural order and electronic properties.

Summary The development of new environmentally friendly luminescent materials is crucial for future solid-state lighting, sensor, and display applications. Here, a Cu(I)-based all-inorganic rare-earth halide material, Rb 8 CuSc 3 Cl 18 , has been synthesized by a solid-state reaction method. In this compound, two Cu(I) ions are connected to three rare-earth halide octahedra to form a paddle-wheel-like cluster. The Cu(I) coordinated rare-earth halide clusters contribute to a strong blue photoluminescence emission. This Cu(I)-regulated emission can be extended to other isostructural compounds, such as Rb 8 CuY 3 Cl 18 . Moreover, the crucial role of Cu(I) has been illustrated by the isostructural non-emissive Rb 8 AgSc 3 Cl 18 . On the basis of comprehensive spectroscopy studies and density functional theory calculations, we found that Cu(I) photo-oxidation and correct orbital-energy-level alignment are crucial for the observed bright-blue emission through a proposed metal (Cu)-to-octahedra ([ScCl 6 ] 3− ) charge-transfer mechanism. The discovery of Cu(I)-based all-inorganic rare-earth halide clusters establishes a new strategy for constructing promising emissive halide materials.

Recently, many high-throughput calculation materials databases have been constructed and found wide applications. However, a database is only useful if its content is reliable and sufficiently accurate. It is thus of paramount importance to gauge the reliabilities and accuracies of these databases. Although many properties have been predicted accurately in these databases, electronic band gap is well known to be underestimated by traditional density functional theory (DFT) calculations under local density approximation (LDA), which becomes a challenging problem for materials database building. Here, we introduce MaterialGo ( http://www.pkusam.com/data-base.html ), a new database calculating the band structures of crystals using both Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional and Heyd-Scuseria-Ernzerhof (HSE) hybrid functional. Comparing different PBE databases, it is found that their band gaps are consistent when no U parameter is used for transition metal d-state or heavy element f-state to correct their self-interaction error, but rather different when PBE+ U are used, mostly because of the different values of U used in different database. HSE calculations under standard parameters will give larger band gaps that are closer to experiment. Based on the high-throughput HSE calculations over 10000 crystal structures, we might have a better understanding of the relationship between crystal structures and electronic structures, which will help us to further explore material genome science and engineering.

Searching for highly efficient, stable and cost-effective catalysts for oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) is vital to resolve energy security and environmental problems. Herein, by means of computational screening based on the density functional theory (DFT), we studied a wide range of transition metal (TM) atoms embedded into the double carbon vacancy of C3N monolayers (VCC), denoted as TM-VCC (TM = Mn, Fe, Co, Ni, Cu, Ru, Rh, Pd, Ir and Pt), as efficient single-atom catalysts (SACs) for OER and ORR. The calculated results showed that all the considered TM-VCC composites exhibited a metallic feature that ensured efficient charge transfer during reactions. The interaction strength between intermediates and TM-VCC has direct correlation with the d-band center of TM, which can be tuned by changing the TM atoms with different numbers of d electrons. The best catalyst for OER was Rh-VCC with an overpotential (ηOER) of 0.35 V, followed by Co-VCC (0.43 V). For the ORR process, Rh-VCC exhibited the lowest ORR overpotential (ηORR) of 0.27 V, followed by Co-VCC (0.42 V). The results suggested that the performances of the newly predicted Rh-VCC and Co-VCC SACs are comparable to those of the noble-metal benchmark catalysts for OER and ORR. Ab initio molecular dynamic simulations indicated that Rh-VCC and Co-VCC SACs could remain stable under 300 K and possessed high energy barriers to prevent the isolated Rh and Co atoms from clustering. Our results highlight a new family of efficient and stable catalysts with a single atom anchored on carbon nitride-based materials, which provides a useful guideline for catalyst design and practical applications.

Recent experiments have shown that CO reduction on oxide derived Cu nanoparticles (NP) are highly selective toward C2+ products. However, understanding of the active sites on such NPs is limited, because the NPs have ∼200 000 atoms with more than 10 000 surface sites, far too many for direct quantum mechanical calculations and experimental identifications. We show here how to overcome the computational limitation by combining multiple levels of theoretical computations with machine learning. This approach allows us to map the machine learned CO adsorption energies on the surface of the copper nanoparticle to construct the active site visualization (ASV). Furthermore, we identify the structural criteria for optimizing selective reduction by predicting the reaction energies of the potential determining step, ΔEOCCOH, for the C2+ product. Based on this structural criterion, we design a new periodic copper structure for CO reduction with a theoretical faradaic efficiency of 97%.

In this paper, we demonstrate that it is practical to use plane-wave based density functional theory (DFT) calculations to study a wide range of problems related to electronic devices like field-effect transistor (FET). Realistic crystalline-Si and amorphous-SiO2 interface is constructed to model the device. A divide and conquer linear scaling three-dimensional fragment method (LS3DF) together with a special Poisson solution scheme is used to solve the whole device system self-consistently under nonequilibrium condition. A special fully ab initio quantum transport calculation method is developed to simulate the current flow through the device. We also show that the plane-wave DFT is capable of studying the reliability dynamics rooted in oxide/interfacial defects including the Si-H bond breaking due to electron excitation.

The widely used density functional theory (DFT) has a major drawback of underestimating the band gaps of materials. Wannier–Koopmans method (WKM) was recently developed for band gap calculations with accuracy on a par with more complicated methods. WKM has been tested for main group covalent semiconductors, alkali halides, 2D materials, and organic crystals. Here we apply the WKM to another interesting type of material system: the transition metal (TM) oxides. TM oxides can be classified as either with d0 or d10 closed shell occupancy or partially occupied open shell configuration, and the latter is known to be strongly correlated Mott insulators. We found that, while WKM provides adequate band gaps for the d0 and d10 TM oxides, it fails to provide correct band gaps for the group with partially occupied d states. This issue is also found in other mean-field approaches like the GW calculations. We believe that the problem comes from a strong interaction between the occupied and unoccupied d-state Wannier functions in a partially occupied d-state system. We also found that, for pseudopotential calculations including deep core levels, it is necessary to remove the electron densities of these deep core levels in the Hartree and exchange–correlation energy functional when calculating the WKM correction parameters for the d-state Wannier functions.

Author(s): Liu, WH; Luo, JW; Li, SS; Wang, LW | Abstract: Photoinduced phase transitions can have complex and intriguing behaviors more than material ground-state dynamics. Understanding the underlying mechanism can help us to design new ways to manipulate the materials. A variety of mechanisms has been proposed to explain the photoinduced phase transitions of IrTe2, but a consensus has yet to be reached. Here, we study the photo-induced phase transitions of IrTe2 by performing the real-time time-dependent density functional theory (rt-TDDFT) simulations in combination with the occupation constrained DFT method. We reveal that the microscopic driving force for the photo-induced phase transitions arises from the tendency to lower the energy levels occupied by the excited carriers, through the increase or decrease of the associated atomic pair distances, depending on whether the newly occupied states are antibonding or bonding states, respectively. The geometric constraints between different bonds represented by the Poisson ratio can bring together different tendencies from different atomic pairs, thus forming a complex intriguing dynamic picture depending on the intensity of the excitation. We also find that phonons don't play a primary role, but can assist the phase transition. These findings imply that one can control the structural phase transitions by selectively exciting photocarriers into designated atomic states using appropriate photon sources.

Real time time dependent density functional theory (rt-TDDFT) has now been used to study a wide range of problems, from optical excitation, to charge transfer, to ion collision, to ultrafast phase transition. However, conventional rt-TDDFT Ehrenfest dynamics for nuclear movement lacks a few critical features to describe many problems: the detail balance between state transition, decoherence for the wave function evolution, and stochastic branching of the nuclear trajectory. There are many-body formalisms to describe such nonadiabatic molecular dynamics, especially the ones based on mixed quantum/classical simulations, like the surface hopping and wave function collapsing schemes. However, there are still challenges to implement such many-body formalisms to the rt-TDDFT simulations, especially for large systems where the excited state electronic structure configuration space is large. Here we introduce two new algorithm for nonadiabatic rt-TDDFT simulations: the first is a Boltzmann factor algorithm which introduces decoherence and detailed balance in the carrier dynamics, but uses mean field theory for nuclear trajectory. The second is a natural orbital branching (NOB) formalism, which use time dependent density matrix for electron evolution, and natural orbital to collapse the wave function upon. It provides decoherence, detailed balance and trajectory branching properties. We have tested these methods for a molecule radiolysis decay problem. We found these methods can be used to study such radiolysis problem in which the molecule is broken into many fragments following complex electronic structure transition paths. The computational time of NOB is similar to the original plain rt-TDDFT simulations

Nonradiative carrier recombinations at deep centers in semiconductors are of great importance for both fundamental physics and device engineering. In this article, we provide a revised analysis of Huang’s original nonradiative multi-phonon (NMP) theory with ab initio calculations. First, we confirmed at the first-principles level that Huang’s concise formula gives the same results as the matrix-based formula, and that Huang’s high-temperature formula provides an analytical expression for the coupling constant in Marcus theory. Secondly, we correct for anharmonic effects by taking into account local phonon-mode variations for different charge states of a defect. The corrected capture rates for defects in GaN and SiC agree well with experiments.

Low-energy impurity diffusion in a host material is often regarded as an adiabatic process, characterized by its adiabatic potential energy barrier. Here we show that the diffusion process in semiconductors can involve nonadiabatic electron excitations, rending it to be a more complicated process. Impurity diffusion in a device at working temperature can pump one electron up from localized impurity state into the host conduction band and causes the impurity to be a dynamic donor since it temporarily loses its electron to the host. This nonadiabatic process, against a common belief, fundamentally change the diffusion behavior, including its barrier height and diffusion path. Although we mainly demonstrate this process with Au metal impurity in bulk Si through time-dependent density functional theory simulations, we believe this could be a rather common phenomenon as it is shown that the similar phenomena also exist in Zn, Cd impurities diffusion in bulk Si, and Ti diffusion in $\mathrm{Ti}{\mathrm{O}}_{2}$. We believe this study can open up a new direction of inquiry for such diffusion behavior in semiconductor.

Theoretical study of the electronic structures of protein is a fundamental challenge in computational biochemistry due to the large size of the systems. The electronic structure of a protein is important for some of the important protein functionalities, such as photosynthesis. In this study, we explored the charge-patching method to calculate the electronic structure of polypeptides. This method generates the charge densities of the systems by patching the charge motifs calculated from small prototype systems. The method was tested on a range of polypeptides, including the glycine polypeptide in 27-ribbon, α-helix, 310-helix, and β-strand structures. After the charge density profiles of these systems were obtained, the electronic structures of these glycine polypeptides were further calculated based on density functional theory (DFT) using a folded-spectrum method. The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) were analyzed and compared with conventional direct DFT calculations. The charge-patching method results were found to be in good agreement with the directed DFT results.

As the high throughout calculations and material genome approaches become more and more popular in material science, the search for optimal ways to predict atomic global minimum structure is a high research priority. This paper presents a fast method for global search of atomic structures at ab initio level. The structures global optimization (SGO) engine consists of a high-efficiency differential evolution algorithm, accelerated local relaxation methods and a plane-wave density functional theory code running on GPU machines. The purpose is to show what can be achieved by combining the superior algorithms at the different levels of the searching scheme. SGO can search the global-minimum configurations of crystals, two-dimensional materials and quantum clusters without prior symmetry restriction in a relatively short time (half or several hours for systems with less than 25 atoms), thus making such a task a routine calculation. Comparisons with other existing methods such as minima hopping and genetic algorithm are provided. One motivation of our study is to investigate the properties of magnetic systems in different phases. The SGO engine is capable of surveying the local minima surrounding the global minimum, which provides the information for the overall energy landscape of a given system. Using this capability we have found several new configurations for testing systems, explored their energy landscape, and demonstrated that the magnetic moment of metal clusters fluctuates strongly in different local minima.

As the characteristic lengths of advanced electronic devices are approaching the atomic scale, ab initio simulation method, with full consideration of quantum mechanical effects, becomes essential to study the quantum transport phenomenon in them. The widely used non-equilibrium Green’s function (NEGF) combined with the density functional theory (DFT) approach prefers a localized basis set. As many states of the art DFT calculations for solid state systems are carried out in plane waves, it is thus worth to investigate the feasibility of using the plane wave basis set for large-scale quantum transport calculations. Here we present a plane wave method for large-scale transport calculations based on a previously developed scattering state calculation approach (Wang, 2005). We address the unique computational challenges of applying that approach for large-scale systems where it is too expensive to calculate all the occupied eigenstates of the system as in conventional DFT calculations. By applying several high-efficiency parallel algorithms, including linear-scaling DFT algorithm, folded spectrum method, and Chebyshev filter technique, we demonstrate that it is possible to use this approach to simulate a system with several thousand atoms on high performance computing platforms. This method is not only used to study nanowire interconnects, showing how the shape and point defect affects their transport properties, but also used to study nanoscale Si transistor. Such quantum transport simulation method will be useful for investigating and designing nanoscale devices.

While a large variety of organic and molecular materials have been found to exhibit charge memory effects, the underlying mechanism is not well-understood, which hinders rational device design. Here, we study the charge retention mechanism of a nanoscale memory system, an organic monolayer on a silicon substrate, with Au nanoparticles on top serving as the electrical contact. Combining scanning probe imaging/manipulation and density functional simulations, we observe stable charge retention effects in the system and attributed it to polaron effects at the amine functional groups. Our findings can pave the way for applications in nonvolatile memory, neuromorphic computing, and high dielectric breakdown devices. U.S. Department of Energy DE-AC02-05CH11231 U.S. National Science Foundation CHE-1300180 Marie Curie FP7 ILSES 612620 Nanotwinning FP7 NN294952

We investigate laser-induced ultrafast demagnetization of ferromagnetic systems for high-speed data processing and storage., Despite more than 20 years of development, the underlying physics of the laser-induced demagnetization process is still debated. We present a fast, real-time time-dependent density functional theory (rt-TDDFT) algorithm together with the phenomenological atomic Landau-Lifshitz-Gilbert model to investigate this problem. Our Hamiltonian considers noncollinear magnetic moment, spin-orbit coupling (SOC), electron-electron, electron-phonon, and electron-light interactions. The algorithm for time evolution achieves hundreds of times of speedup enabling calculation of large systems. Our simulations yield a demagnetization rate similar to experiments. We found that (i) the angular momentum flow from light to the system is not essential and the spin Zeeman effect is negligible. (ii) The phonon can play a role but is not essential. (iii) The initial spin disorder and the self-consistent update of the electron-electron interaction play dominant roles and enhance the demagnetization to the experimentally observed rate. The spin disorder connects the electronic structure theory with the phenomenological three-temperature model.

Real-time time-dependent density functional theory (rt-TDDFT) with hybrid exchange-correlation functional has wide-ranging applications in chemistry and material science simulations. However, it can be thousands of times more expensive than a conventional ground state DFT simulation, and hence is limited to small systems. In this paper, we accelerate hybrid functional rt-TDDFT calculations using the parallel transport gauge formalism, and the GPU implementation on Summit. Our implementation can efficiently scale to 786 GPUs for a large system with 1536 silicon atoms, and the wall clock time is only 1.5 hours per femtosecond. This unprecedented speed enables the simulation of large systems with more than 1000 atoms using rt-TDDFT and hybrid functional.

In order to develop force fields (FF) for molecular dynamics simulations that retain the accuracy of ab initio density functional theory (DFT), we developed a machine learning protocol based on an energy decomposition scheme that extracts atomic energies from DFT calculations. Our DFT to FF (DFT2FF) approach provides almost hundreds of times more data for the DFT energies, which dramatically improves accuracy with less DFT calculations. In addition, we use piecewise cosine basis functions to systematically construct symmetry invariant features into the neural network model. We illustrate this DFT2FF approach for amorphous silicon where only 800 DFT configurations are sufficient to achieve an accuracy of 1 meV/atom for energy and 0.1 eV/A for forces. We then use the resulting FF model to calculate the thermal conductivity of amorphous Si based on long molecular dynamics simulations. The dramatic speedup in training in our DFT2FF protocol allows the adoption of a simulation paradigm where an accurate and problem specific FF for a given physics phenomenon is trained on-the-spot through a quick DFT precalculation and FF training.

Real-time time-dependent density functional theory (RT-TDDFT) is known to be hindered by the very small time step (attosecond or smaller) needed in the numerical simulation, because of the fast oscillation of electron wave functions, which significantly limits its range of applicability for the study of ultrafast dynamics. In this paper, we demonstrate that such oscillation can be considerably reduced by optimizing the gauge choice using the parallel transport formalism. RT-TDDFT calculations can thus be significantly accelerated using a combination of the parallel transport gauge and implicit integrators, and the resulting scheme can be used to accelerate any electronic structure software that uses a Schrodinger representation. Using absorption spectrum, ultrashort laser pulse, and Ehrenfest dynamics calculations for example, we show that the new method can utilize a time step that is on the order of 10-100 attoseconds using a planewave basis set. Thanks to the significant increase of the size of the time step, we also demonstrate that the new method is more than 10 times faster, in terms of the wall clock time, when compared to the standard explicit fourth-order Runge-Kutta time integrator for silicon systems ranging from 32 to 1024 atoms.

Author(s): Gao, G; Zheng, F; Pan, F; Wang, LW | Abstract: Even though tremendous achievement has been made experimentally in the performance of lithium–sulfur (Li–S) battery, theoretical studies in this area are lagging behind due to the complexity of the Li–S systems and the effects of solvent. For this purpose, a new methodology is developed for investigating the 2D hexaaminobenzene-based coordination polymers (2D-HAB-CPs) as cathode candidate materials for Li–S batteries via density functional theory calculations in combination with an in-house developed charge polarized solvent model and a genetic algorithm structure global search code. With high ratios of transition metal atoms and two-coordinated nitrogen atoms, excellent electric conductivity, and structural porosity, the 2D-HAB-CP is able to address all of the three main challenges facing Li–S batteries: confining the lithium polysulfides from dissolution, facilitating the electron conductivity and buffering the volumetric expansion during the lithiation process. In addition, the theoretical energy density of this system is as high as 1395 Wh kg−1. These results demonstrate that the 2D-HAB-CP is a promising cathode material for Li–S batteries. The proposed computational framework not only opens a new avenue for understanding the key role played by solution and liquid electrolytes in Li–S batteries, but also can be generally applied to other processes with liquids involved.

We use the organic semiconductor tetrathiafulvalene dicarboxylate (TTFDA) to assemble PbS nanocrystals into conductive mesocrystals. Density functional theory calculations predict a size-tunable, near-resonant alignment between the PbS 1Sh state and the TTFDA HOMO with the potential to form a conductive channel for holes. We test this hypothesis with transport measurements on TTFDA-functionalized PbS nanocrystals of different sizes and find a pronounced modulation of the field-effect hole mobilities. Photothermal deflection spectroscopy reveals unchanged Urbach energies after ligand exchange, whereas further surface modification by colloidal-atomic layer deposition leads to a strong increase in the density of in-gap states. Hole transport in PbS-TTFDA is unusually robust against such surface modification. Our structural analysis of the mesocrystals suggests that TTFDA induces a defined interparticle spacing, the orientation of atomic lattices, and the angle within the mesocrystal unit cell. The results of...

Artificial solids composed of semiconductor quantum dots (QDs) are being developed for large-area electronic and optoelectronic applications, but these materials often have defect-induced in-gap states (IGS) of unknown chemical origin. Here we performed scanning probe based spectroscopic analysis and density functional theory calculations to determine the nature of such states and their electronic structure. We found that IGS near the valence band occur frequently in the QDs except when treated with reducing agents. Calculations on various possible defects and chemical spectroscopy revealed that molecular oxygen is most likely at the origin of these IGS. We expect this impurity-induced deep IGS to be a common occurrence in ionic semiconductors, where the intrinsic vacancy defects either do not produce IGS or produce shallow states near band edges. Ionic QDs with surface passivation to block impurity adsorption are thus ideal for high-efficiency optoelectronic device applications.

A major drawback of the widely successful density functional theory is its underestimation of the material band gap. Various methods have been proposed to correct its band gap predictions. Wannier Koopmans method (WKM) is recently developed for this purpose to predict the band gap of extended 3D bulk systems. While the WKM has also been shown to be successful for isolated molecules, it is still a question whether it will work for 2D materials that are in between the 0D molecules and 3D bulk systems. We apply the WKM to 16 commonly known well studied 2D materials and find that the WKM predicted band gaps are on par with their GW calculated results.

We present an ab initio method to calculate elastic quantum transport at the nanoscale. The method is based on a combination of density functional theory using plane wave nonlocal pseudopotentials and the use of auxiliary periodic boundary conditions to obtain the scattering states. The method can be applied to any applied bias voltage and the charge density and potential profile can either be calculated self-consistently, or using an approximated self-consistent field (SCF) approach. Based on the scattering states one can straightforwardly calculate the transmission coefficients and the corresponding electronic current. The overall scheme allows us to obtain accurate and numerically stable solutions for the elastic transport, with a computational time similar to that of a ground state calculation. This method is particularly suitable for calculations of tunneling currents through vacuum, that some of the nonequilibrium Greens function (NEGF) approaches based on atomic basis sets might have difficulty to deal with. Several examples are provided using this method from electron tunneling, to molecular electronics, to electronic devices: (i) On a Au nanojunction, the tunneling current dependence on the electrode–electrode distance is investigated. (ii) The tunneling through field emission resonances (FERs) is studied via an accurate description of the surface vacuum states. (iii) Based on quantum transport calculations, we have designed a molecular conformational switch, which can turn on and off a molecular junction by applying a perpendicular electric field. (iv) Finally, we have used the method to simulate tunnel field-effect transistors (TFETs) based on two-dimensional transition-metal dichalcogenides (TMDCs), where we have studied the performance and scaling limits of such nanodevices and proposed atomic doping to enhance the transistor performance.

The amorphous titanium dioxide (a-TiO2) has drawn attention recently due to the finding that it holds promise for coating conventional photoelectrodes for corrosion protection while still allowing the holes to transport to the surface. The mechanism of hole conductivity at a level much higher than the edge of the valence band is still a mystery. In this work, an amorphous TiO2 model is obtained from molecular dynamics employing the "melt-and-quench" technique. The electronic properties, polaronic states and the hole conduction mechanism in amorphous structure were investigated by means of density functional theory with Hubbard's energy correction (DFT + U) and compared to those in crystalline (rutile) TiO2. The formation energy of the oxygen vacancy was found to reduce significantly (by a few eV) upon amorphization. Our theoretical study suggested that the oxygen vacancies and their defect states provide hopping channels, which are comparable to experimental observations and could be responsible for hole conduction in the "leaky" TiO2 recently discovered for the photochemical water-splitting applications.

Computational studies have been performed to elucidate the activation mechanism of the Michael addition reactions containing bifunctional tertiary amine–thioureas and isatylidene malononitriles by density functional theory (DFT) calculations at the B3LYP/6-311++G(d,p)//B3LYP/6-31G(d) level of theory. Results showed a difference of 6.47 kcal mol−1 between M1-O and M1-N, which suggest that it is the carbonyl group, instead of the malononitrile moiety of isatylidene malononitriles, that plays a dominating role in the activation of the electrophile by the catalysts. The predicted mechanism also successfully explains the experimentally observed enantioselectivity.

The regioselective and enantioselective Michael addition between azlactones and o-hydroxy chalcone derivatives is reported. Enantiomerically enriched N,O-aminals with two continuous stereogenic centers are exclusively obtained in moderate to good yields with excellent diastereoselectivities and good to excellent enantioselectivities. The experimental results show that an o-hydroxy group on the cinnamenyl motif of chalcone derivatives plays a crucial role at the reaction sites for the regioselective Michael addition. In addition, circular dichroism (CD) spectroscopy and density functional theory (DFT) are used to investigate the absolute configuration of N,O-aminals and the corresponding transition-state structures.

A formal thio [3 + 3]-cyclization catalyzed by a DPEN-derived chiral thiourea has been reported for the construction of optically active thiopyrano-indole annulated heterocyclic compounds in high yields with excellent enantioselectivities. The high reactivity between indoline-2-thione (keto-S) and 2-benzylidenemalononitrile has also been supported by density functional theory (DFT) calculations.

The origin of the enantio- and regioselectivity of ring-opening reaction of oxabicyclic alkenes catalyzed by rhodium/Josiphos has been examined using M06-2X density functional theory(DFT). DFT calculations predict a 98% ee for the enantioselectivity and only the 1,2-trans product as one regio- and diastereomer, in excellent agreement with experimental results. The solvent tetrahydrofuran(THF) plays a key role in assisting nucleophilic attack. Orbital composition analysis of the LUMO and the NPA atomic charge calculations were conducted to probe the origins of the regioselectivity. The orbital composition analysis reveals two potential electrophilic sites of the Rh–π-allyl intermediate M3 and the NPA atomic charges demonstrate that Cα carries more positive charges than Cγ, which suggests that Cα is the electrophilic site.

We have investigated the performance of the mono-layer MoS2 tunnel FET (TFET) through quantum transport simulations based on density functional theory (DFT). Two kinds of source/drain metal contact configurations namely the seamless contact and side contact are compared. It was found that ideal band bending control by seamless contact yields much better performance than the side contact, although the device performance is still far from the ITRS expectations of the future high performance (HP) and low power (LP) transistors. In the meanwhile, introduction of mid-gap defects near source channel interface was found to be effective performance booster. Such MoS2 TFET with Mo vacancy defects was found to outperform the ITRS HP requirement.

Plane wave pseudopotential (PWP) density functional theory (DFT) calculation is the most widely used method for material simulations, but its absolute speed stagnated due to the inability to use large scale CPU based computers. By a drastic redesign of the algorithm, and moving all the major computation parts into GPU, we have reached a speed of 12s per molecular dynamics (MD) step for a 512 atom system using 256GPU cards. This is about 20 times faster than the CPU version of the code regardless of the number of CPU cores used. Our tests and analysis on different GPU platforms and configurations shed lights on the optimal GPU deployments for PWP-DFT calculations. An 1800 step MD simulation is used to study the liquid phase properties of GaInP.

It is important to accurately predict the band gaps of crystals, including organic crystals, with low computational cost. Despite the significant underestimation of the crystal band gap by the density functional theory (DFT), a recently proposed Wannier-Koopmans method (WKM) based on DFT calculations seems to yield accurate band gaps for a wide class of materials including common semiconductors, alkali halides and 2D materials. It is nevertheless important to test the limit of WKM, in particular in systems with unique characteristics. In this work, we apply the WKM to 10 organic small molecule crystals and find that the WKM calculated band gaps agree well with GW results. We also introduce a new way to calculate the Wannier functions in the WKM calculations.

The optimization of the atomic and molecular clusters with a large number of atoms is a very challenging topic. This article proposes a parallel differential evolution (DE) optimization scheme for large-scale clusters. It combines a modified DE algorithm with improved genetic operators and a parallel strategy with a migration operator to address the problems of numerous local optima and large computational demanding. Results of Lennard-Jones (LJ) clusters and Gupta-potential Co clusters show the performance of the algorithm surpasses those in previous researches in terms of successful rate, convergent speed, and global searching ability. The overall performance for large or challenging LJ clusters is enhanced significantly. The average number of local minimizations per hit of the global minima for Co clusters is only about 3-4% of that in previous methods. Some global optima for Co are also updated. We then apply the algorithm to optimize the Pt clusters with Gupta potential from the size 3 to 130 and analyze their electronic properties by density functional theory calculation. The clusters with 13, 38, 54, 75, 108, and 125 atoms are extremely stable and can be taken as the magic numbers for Pt systems. It is interesting that the more stable structures, especially magic-number ones, tend to have a larger energy gap between the highest occupied molecular orbital and the lowest unoccupied molecular orbital. It is also found that the clusters are gradually close to the metal bulk from the size N > 80 and Pt38 is expected to be more active than Pt75 in catalytic reaction.

a b s t r a c t Plane wave pseudopotential (PWP) density functional theory (DFT) calculation is the most widely used material science simulation, and the PWP DFT codes are arguably the most important material science codes. We have implemented a PWP DFT code PEtot on a multi-node GPU machine. Starting from a previous work, we have further improved the speed of the code, and achieved x13-x22 speedups over the CPU calculations for a typical 512 atom system. Such speedups are much higher than other similar works for this important class of material simulation codes on GPU clusters. The current achievement is obtained by (1) moving the calculation fully into the GPU; (2) adopting a new algorithm to reduce the data amount for MPI communication; and (3) using new GPU and CPU numerical libraries. We have also provided a detail quantitative analysis of the computational times for different physical systems and number of GPU units, which helps one to understand the challenges and bottlenecks of the PWP DFT simulations on GPU machines. Based on the analysis, we listed the machine and library requirements in order to further improve the performances of the PWP DFT calculations.

© 2016 The Royal Society of Chemistry. We have demonstrated a rational two-dimensional (2D) conjugation approach towards achieving panchromatic absorption of small molecules. By extending the conjugation on two orthogonal axes of an electron acceptor, namely, bay-annulated indigo (BAI), the optical absorptions could be tuned independently in both high- and low-energy regions. The unconventional modulation of the high-energy absorption is rationalized by density functional theory (DFT) calculations. Such a 2D tuning strategy provides novel guidelines for the design of molecular materials with tailored optoelectronic properties.

Orbital-free density functional theory (OFDFT) is a quantum-mechanics-based method that utilizes electron density as its sole variable. The main computational cost in OFDFT is the ubiquitous use of the fast Fourier transform (FFT), which is mainly adopted to evaluate the kinetic energy density functional (KEDF) and electron-electron Coulomb interaction terms. We design and implement a small-box FFT (SBFFT) algorithm to overcome the parallelization limitations of conventional FFT algorithms. We also propose real-space truncation of the nonlocal Wang-Teter KEDF kernel. The scalability of the SBFFT is demonstrated by efficiently simulating one full optimization step (electron density, energies, forces, and stresses) of 1,024,000 lithium (Li) atoms on up to 65,536 cores. We perform other tests using Li as a test material, including calculations of physical properties of different phases of bulk Li, geometry optimizations of nanocrystalline Li, and molecular dynamics simulations of liquid Li. All of the tests yield excellent agreement with the original OFDFT results, suggesting that the OFDFT-SBFFT algorithm opens the door to efficient first-principles simulations of materials containing millions of atoms.

We present large-scale calculations of electronic structure of strongly disordered conjugated polymers. The calculations have been performed using the density functional theory based charge patching method for the construction of single-particle Hamiltonian and the overlapping fragments method for the efficient diagonalization of that Hamiltonian. We find that the hole states are localized due to the fluctuations of the electrostatic potential and not by the breaks in the conjugation of the polymer chain. The tail of the density of hole states exhibits an exponentially decaying behavior. The main features of the electronic structure of the system can be described by an one-dimensional nearest neighbor tight-binding model with a correlated Gaussian distribution of on-site energies and constant off-site coupling elements.