Density Functional Tight-binding Approach Research Articles

Investigating Si (100) surface patterns as well as electrical states through integrating molecular dynamics simulations with density functional tight binding at atomic level

The present work performs molecular dynamics simulations within model potential’s framework to investigate surface atoms’ rearrangements, loading states, and stress distribution in Si (100) surface on heating, and then single point energy calculations for the obtained atomic packing structures of Si substrates are used to obtain electrical information through self-consistent charge density functional tight binding approach. From analysis of Lode-Nadai parameters, atomic stress, visually packing images, Mulliken charges of atoms, Mulliken electron populations on atomic orbitals, differential charge densities, and chemical potentials for substrates, the knowledge of surface patterns, micro-loading states on the atoms and electrical states are obtained. The simulation results show the positions of vacancies have a significant impact on the rearrangements of the surface atoms. The appropriate selection of vacancy positions could be helpful in designing patterns of Si (100) surface. The vacancies in the subsurface or deeper atomic layers present migrations, where the migrations are more likely to occur at higher temperatures. There are stress concentration area and apparent charge transfers for the atoms forming dimers and neighboring atoms around the vacancies, and the differential charge densities in the surface atoms also present significant differences. These substrates with different surface patterns have different chemical potentials.

The electronic transport properties of armchair carbon nanotubes with single and multiple horizontal boron-nitride lines: Unusual effects of high bias voltage on I-V characteristics

We investigate the electronic band structure properties for different arrangements of infinite horizontal boron nitride (BN) lines in the (6,6) armchair carbon nanotube. Depending on the arrangement of the boron and nitrogen atoms, the horizontal BN lines can be viewed as single or multiple BN segments along the contour of the host carbon nanotube. According to the geometrical orientation of the BN segment(s) in the circular direction, the band gap energy can be tuned providing semiconducting properties for the parallel geometries and metallic behaviors for the antiparallel geometries respectively. For the non-equilibrium transport properties, the current-voltage characteristics for such structures are also examined. Conventional fluctuations in the current are obtained for the high bias voltage as a sign of the resonant tunneling effects in the electronic transport process. However, unusual current-voltage characteristics are reported for particular BN segment configurations in the anti-parallel multiple geometry. The transmission functions and the local currents are respectively presented pointing out more details about such discrepancy in both the electronic transport process near the Fermi level and the nature of the corresponding wave functions. All calculations are performed by means of the density functional tight-binding approach as implemented in the DFTB+ software package.

Atomic bonding states of metal and semiconductor elements

In this paper, we use density functional theory (DFT) to calculate the deformation electron density of 46 metal and semiconductor elements. The binding-energy and bond-charge model (BBC) model is combined with the tight-binding and density-functional–tight-binding approaches to obtain quantitative information about atomic bonding at the atomic scale and to understand the contributions and effects of deformation energy density, energy shifts, and atomic bonding on the Hamiltonian. The bonding state is obtained through energy shift and deformation charge density. The BBC model involving no assumptions or freely adjustable parameters, has led to consistency between predictions and experimental observations of the cohesive energy and energy density of nanosolids.

Raman Evidence of Multiple Adsorption Sites and Structural Transformation in ZIF-4

The zeolitic imidazolate framework, ZIF-4, exhibits soft porosity and is known to show pore volume changes with temperatures, pressures, and guest adsorption. However, the mechanism and adsorption behavior of ZIF-4 are not completely understood. In this work, we report an open to narrow pore transition in ZIF-4 around T ∼ 253 K upon lowering the temperature under vacuum (10-6 Torr) conditions, facilitated by C-H···π interactions. In the gaseous environment of N2 and CO2 around the framework, characteristic Raman peaks of adsorbed gases were observed under ambient conditions of 293 K and 1 atm. A guest-induced transition at ∼153 K resulting in the opening of new adsorption sites was inferred from the Raman spectral changes in the C-H stretching modes and low-frequency modes (<200 cm-1). In contrast to a single vibrational mode generally reported for entrapped N2, we show three Raman modes of adsorbed N2 in ZIF-4. The adsorption is facilitated by dispersive and quadrupolar interactions. From our temperature-dependent Raman results and theoretical analysis based on the density functional tight-binding approach, we conclude that the C-Hs are the preferred adsorption sites on ZIF-4 in the following order: C4-H, C5-H > C2-H > center of the Im ring (interacting with C-H centers) > center of the cavity. We also show that with an increasing concentration of N2 adsorbed at low temperatures, the ZIF-4 structure undergoes shear distortion of the window formed by 4-imidazole rings and consequent volumetric expansion. Our results have immediate implications in the field of porous materials and could be vital in identifying subtle structural transformations that may favor or hinder guest adsorption.

DFTBephy: A DFTB-based approach for electron–phonon coupling calculations

The calculation of the electron–phonon coupling from first principles is computationally very challenging and remains mostly out of reach for systems with a large number of atoms. Semi-empirical methods, like density functional tight binding (DFTB), provide a framework for obtaining quantitative results at moderate computational costs. Herein, we present a new method based on the DFTB approach for computing electron–phonon couplings and relaxation times. It interfaces with phonopy for vibrational modes and dftb+ to calculate transport properties. We derive the electron–phonon coupling within a non-orthogonal tight-binding framework and apply them to graphene as a test case.

Origin of metallic-like behavior in disordered carbon nano-onions

Carbon nano-onions are a class of nanomaterials that can exhibit long electron spin relaxation times at room temperature and thus hold promise as potential building blocks for spintronics and quantum information processing devices. Despite first being synthesized 30 years ago, there exists a gap in understanding electronic and magnetic properties of these nanostructures. Here we investigate the origin of the metallic-like behavior that has been observed experimentally in disordered nano-onions. Employing a density functional tight-binding approach and starting from ordered multi-shell fullerenes, we develop realistic models of highly disordered nano-onions comprised of nanometer-scale graphitic flakes. We find that multiple parameters such as flake size, edge structure and substituent groups can give rise to in-gap metallic states, effectively closing the HOMO-LUMO gap.

GPU-Enhanced DFTB Metadynamics for Efficiently Predicting Free Energies of Biochemical Systems.

Metadynamics calculations of large chemical systems with ab initio methods are computationally prohibitive due to the extensive sampling required to simulate the large degrees of freedom in these systems. To address this computational bottleneck, we utilized a GPU-enhanced density functional tight binding (DFTB) approach on a massively parallelized cloud computing platform to efficiently calculate the thermodynamics and metadynamics of biochemical systems. To first validate our approach, we calculated the free-energy surfaces of alanine dipeptide and showed that our GPU-enhanced DFTB calculations qualitatively agree with computationally-intensive hybrid DFT benchmarks, whereas classical force fields give significant errors. Most importantly, we show that our GPU-accelerated DFTB calculations are significantly faster than previous approaches by up to two orders of magnitude. To further extend our GPU-enhanced DFTB approach, we also carried out a 10 ns metadynamics simulation of remdesivir, which is prohibitively out of reach for routine DFT-based metadynamics calculations. We find that the free-energy surfaces of remdesivir obtained from DFTB and classical force fields differ significantly, where the latter overestimates the internal energy contribution of high free-energy states. Taken together, our benchmark tests, analyses, and extensions to large biochemical systems highlight the use of GPU-enhanced DFTB simulations for efficiently predicting the free-energy surfaces/thermodynamics of large biochemical systems.

First-Principles Nonequilibrium Green's Function Approach to Energy Conversion in Nanoscale Optoelectronics.

Understanding photon-electron conversion on the nanoscale is essential for future innovations in nano-optoelectronics. In this article, based on nonequilibrium Green's function (NEGF) formalism, we develop a quantum-mechanical method for modeling energy conversion in nanoscale optoelectronic devices. The method allows us to study photoinduced charge transport and electroluminescence processes in realistic devices. First, we investigate the electroluminescence properties of a two-level model with two different treatments of inelastic scatterings. We show the regime where self-consistency between electron and photon is important for correct description of the inelastic scatterings. The method is then applied to model single-molecule junctions based on the density-functional tight-binding approach. The predicted emission spectra are found to be in very good agreement with experimental measurements. For nanostructured materials, the method is further applied to study the photoresponse of a two-dimensional graphene/graphite-C3N4 heterojunction photovoltaic device. The simulations demonstrate clearly the impact of atomistic details on the optoelectronic properties of nanodevices. This work provides a practical theoretical framework that can be applied to model and design realistic nanodevices.

Probing the range of applicability of structure- and energy-adjusted QM/MM link bonds II: Optimized link bond parameters for density functional tight binding approaches.

Optimized link bond parameters for the Cα—Cβ bond of 22 different capped amino acid model systems have been determined at SCC DFTB/mio (self‐consistent charge density functional tight‐binding), SCC DFTB/3ob and GFNn‐xTB (n = 0, 1, and 2) level in conjunction with the AMBER 99SB, 14SB, and 19B force fields. The resulting parameter sets have been compared to newly calculated reference data obtained via resolution‐of‐identity 2nd order Møller–Plesset perturbation theory. The data collected in this work suggests that the optimized values in this study provide a more suitable setup of the QM/MM link bonds compared to the use of a single global setting applied to every amino acid fragmented by the QM/MM interface. The results also imply that a transfer of the ideal link bond settings between different levels of theory is not advised. In contrast, virtually identical parameters were obtained in calculations employing different variants of the AMBER force field. Considering the increasing success of tight binding based approaches being inter alia a results of their exceptional accuracy/effort ratio the provided collection of link atoms parameters provides a valuable resource for QM/MM studies of biomacromolecular systems as demonstrated in an exemplary QM/MM MD simulation of the β‐amyloid/Zn2+ complex.

Density functional tight binding approach utilized to study X-ray-induced transitions in solid materials

Intense X-ray pulses from free-electron lasers can trigger ultrafast electronic, structural and magnetic transitions in solid materials, within a material volume which can be precisely shaped through adjustment of X-ray beam parameters. This opens unique prospects for material processing with X rays. However, any fundamental and applicational studies are in need of computational tools, able to predict material response to X-ray radiation. Here we present a dedicated computational approach developed to study X-ray induced transitions in a broad range of solid materials, including those of high chemical complexity. The latter becomes possible due to the implementation of the versatile density functional tight binding code DFTB+ to follow band structure evolution in irradiated materials. The outstanding performance of the implementation is demonstrated with a comparative study of XUV induced graphitization in diamond.

Studies of Molecular Dynamics and Electronic Structure in Cubic-SiC by Using Density Functional Tight Binding Approach

The radiation damage in Silicon-Carbide and its result are presented in this research. The Density Functional Tight Binding (DFTB) approach is used to perform molecular dynamics simulations to implement the DFTB+ code (Elstner et al., 1998). This methodology shows the making and breaking of chemical bonding as well as describes the realistic total energy for the larger systems. Repulsive potentials have been developed to prevent the atoms to stay close to each other during the model of high energy collisions also correctly describe the configurations during the atomic separation within the typical range. The extent and nature of damages are characterized within the collision event up to 10KeV. The band structure of SiC has been studied using minimal basis (sp) as applied in DFTB+. The value of band gap shows that cubic SiC is a large band gap semiconductor material. This value is comparable with the given in literature. Density of states is also calculated by using the tight binding approach. The peaks of spectrum have been compared with the experimental values found in literature.

Impact of nitrogen doping on the linear and nonlinear terahertz response of graphene

It is well known that impurities play a central role in the linear and nonlinear response of graphene at optical and terahertz frequencies. In this work, we calculate the bands and intraband dipole connection elements for nitrogen-doped monolayer graphene using a density functional tight-binding approach. Employing these results, we calculate the linear and nonlinear response of the doped graphene to terahertz pulses using a density-matrix approach in the length gauge. We present the results for the linear and nonlinear mobility as well as third-harmonic generation in graphene for adsorbed and substitutional nitrogen doping for a variety of doping densities. We show that the conduction bands are more parabolic in graphene structures with substitutional nitrogen doping than for those with adsorbed nitrogen. As a result, substitutional doping has a greater impact on the terahertz mobility and nonlinear response of graphene than adsorbed nitrogen does.

Fast Nonadiabatic Molecular Dynamics via Spin-Flip Time-Dependent Density-Functional Tight-Binding Approach: Application to Nonradiative Relaxation of Tetraphenylethylene with Locked Aromatic Rings.

Nonadiabatic dynamics around conical intersections between ground and excited states are crucial to understand excited-state phenomena in complex chemical systems. With this background in mind, we present an approach combining fewest-switches trajectory surface hopping and spin-flip (SF) time-dependent (TD) density-functional tight binding (DFTB), which is a simplified version of SF-TD density functional theory (DFT) with semiempirical parametrizations, for computationally efficient nonadiabatic molecular dynamics simulations. The estimated computational time of the SF-TD-DFTB approach is several orders of magnitude lower than that of SF-TD-DFT. In addition, the proposed method reproduces the time scales and quantum yields in photoisomerization reactions of azobenzene at a level comparable with conventional ab initio approaches, demonstrating reasonable accuracy. Finally, we report a practical application of the developed technique to explore the nonradiative relaxation processes of tetraphenylethylene and its derivative with torsionally locked aromatic rings and discuss the effect of locking the rings on the excited-state lifetime.

Density Functional Tight Binding Theory Approach for the CO2 Reduction Reaction Paths on Anatase TiO2 Surfaces.

Herein, we have investigated the CO2 reduction paths on the (101) anatase TiO2 surface using an approach based on the density functional tight binding (DFTB) theory. We analyzed the reaction paths for the conversion of carbon dioxide to methane by performing a large number of calculations with intermediates placed in various orientations and locations at the surface. Our results show that the least stable intermediate is CO2H and therefore a key bottleneck is the reduction of CO2 to formic acid. Hydrogen adsorption is also weak and would also be a limiting factor, unless very high pressures of hydrogen are used. The results from our DFTB approach are in good agreement with the hybrid functional based density functional theory calculations presented in the literature.

Performance study of the electronic and optical parameters of thermally activated delayed fluorescence nanosized emitters (CCX-I and CCX-II) via DFT, SCC-DFTB and B97-3c approaches

Conventional density functional theory (DFT) is the common choice of researchers. However, recently the density functional tight binding (DFTB) methods and composite B97-3c methods have been used as a faster and accurate DFT alternative. Moreover, the B97-3c based time-dependent DFT, simplified Tamm–Dancoff (sTDA) and simplified TDDFT (sTDDFT) calculations have expanded the domain of state of art excited state calculations. Here, the recently developed organic nanosized emitters 3-(9H-[3,9′-bicarbazol]-9-yl)-9H-xanthen-9-one (CCX-I) and 3-(9′H-[9,3′:6′,9″-tercarbazol]-9′-yl)-9H-xanthen-9-one (CCX-II) have been used as samples to study the performance of the above mentioned methods at the ground and excited levels in gas and solvent conditions, both with and without the dispersion corrections respectively. Bond lengths calculated by DFTB and B97-3c approaches are in excellent agreement with the DFT results. Root mean square deviation (RMSD) values suggest that the DFTB approach provides better band gap values than B97-3c. But, B97-3c results in better torsion angles. Optical properties are found to be closest to the DFT values when calculated by time-dependent DFTB (TD-DFTB) approach with the lowest RMSDs for excitation energies (0.75 eV) and oscillator strengths (~0.08). The computational speed boosts of the B97-3c based sTDDFT and sTDA approaches are ~ 4 and ~33 times faster than the TDDFT counterpart.

Density-functional tight-binding approach for the structural analysis and electronic structure of copper hydride metallic nanoparticles

We perform a theoretical investigation using the density functional tight-binding (DFTB) approach for the structural analysis and electronic structure of copper hydride (CuH) metallic nanoparticles (NPs) of different size (from 0.7 to 1.6 nm). By increasing the size of CuH NPs, the number of bonds, segregation phenomena and radial distribution function (RDF) of binary Cu-Cu, Cu-H and HH interactions are analyzed using new implementations in R code. The results reveal that the number of Cu-Cu bonds is more than that of Cu-H while the number of HH bonds are the less. Thus, a large amount of H atoms prefers to connect to Cu atoms. The increase in the size of the NPs contributes to their stabilization because of the increase in the interaction of HH bonding. The segregation of Cu and H atoms shows that Cu atoms tend to co-locate at the center, while H atoms tend to reside on the surface. From the density of state (DOS) analysis, CuH NPs shows a metallic character which is compatible with experimental data. HOMO and Fermi levels decrease from -3.555 to -3.443 eV and from -3.510 to -3.441 eV. Herein, an increase in the size contributes to the stabilization of CuH NP due to decrease in the HOMO energies.

Comparison between density functional theory and density functional tight binding approaches for finding the muon stopping site in organic molecular crystals.

Finding the possible stopping sites for muons inside a crystalline sample is a key problem of muon spectroscopy. In a previous study, we suggested a computational approach to this problem when dealing with muonium, the pseudoatom formed by a positive muon that has captured an electron, using density functional theory software in combination with a random structure searching approach that relies on a Poisson sphere distribution. In this work, we test this methodology further by applying it to muonium in three organic molecular crystal model systems: durene, bithiophene, and tetracyanoquinodimethane. Using the same sets of random structures, we compare the performance of density functional theory software CASTEP and the much faster lower level approximation of Density Functional Tight Binding provided by DFTB+ combined with the use of the 3ob-3-1 parameter set. We show the benefits and limitations of such an approach, and we propose the use of DFTB+ as a viable alternative to more cumbersome simulations for routine site-finding in organic materials. Finally, we introduce the Muon Spectroscopy Computational Project software suite, a library of Python tools meant to make these methods standardized and easy to use.

Formulation and Implementation of the Spin-Restricted Ensemble-Referenced Kohn-Sham Method in the Context of the Density Functional Tight Binding Approach.

The spin-restricted ensemble-referenced Kohn-Sham (REKS) method and its state-interaction state-averaged variant (SI-SA-REKS, or SSR) provide computational platform for seamless inclusion of multireference effects into the density functional calculations. The SSR method enables an accurate calculation of the vertical excitation energies for the molecules with multireference ground states and describes conical intersections between the ground and excited states with the accuracy matching the most sophisticated ab initio multireference wave function methods. In this work, the SSR method is formulated and implemented in the context of the long-range corrected density functional tight binding (LC-DFTB) approach. The new LC-DFTB/SSR method enables calculation of the excited electronic states and the S1/S0 conical intersections of very large molecules. The LC-DFTB/SSR method is benchmarked against vertical excitation energies and conical intersection energies and geometries of several organic molecules with π/π* and n/π* transitions. It is demonstrated that the LC-DFTB/SSR method describes these molecules with reasonable accuracy, which can be considerably improved by a slight modification of the LC-DFTB spin polarization parameters.

A bulk adjusted linear combination of atomic orbitals (BA-LCAO) approach for nanoparticles.

We describe a bulk adjusted linear combination of atomic orbitals (BA-LCAO) approach for nanoparticles. In this method, we apply a many-body scaling function (in similar manner as in the environment-modified total energy based tight-binding method) to the DFT-derived diatomic AO interaction potentials (like in the conventional orbital-based density-functional tight binding approach) strictly according to atomic valences acquired naturally in a bulk structure. This modification, (a) facilitates all atom orbital-based electronic structure calculations of charge carrier dynamics in nanoscale structures with a molecular acceptor, and (b) allows to closely match high-level density functional calculation data (previously adjusted to the available experimental findings) for bulk structures. To advance practical application of the BA-LCAO approach we parameterize the Hamiltonian of wurtzite CdSe by fitting its band structure to a high-level DFT reference, corrected for experimentally measured band edges. Here, unlike in conventional DFTB approach, we: (1) use hydrogen-like AOs for the basis as exact atomic eigenfunctions, while orbital energies of which are taken from experimentally measured ionization potentials, and (2) parameterize the many-body scaling functions rather than the atomic wavefunctions. Development of this approach and parameters is guided by our goals to devise a method capable of simultaneously treating the problems of (i) interfacial electron/hole transfer between finite, variable size nanoparticles and electron scavenging molecules, and (ii) high-energy electronic transitions (Auger transitions) that mediate multi-exciton decay in quantum dots. Electronic structure results are described for CdSe quantum dots of various sizes. © 2018 Wiley Periodicals, Inc.

Adsorption on a nanoporous organic polymer for clean energy applications: A multiscale modeling study using density functional tight binding approach

Adsorption of carbon dioxide, methane and hydrogen on the novel ferrocenyl-nanoporous organic polymer was studied from an approximate density-functional method. Geometric, energetic and electronic parameters are obtained using the self-consistent charge density functional tight binding (SCC-DFTB) spin polarized approach and prove a mechanism of adsorption governed by physisorption processes. In order to gain a deeper understanding of the effect of temperature on the adsorption-desorption process, an SCC-DFTB/MD simulation is implemented which enabled a satisfactory agreement with the experimental results reported in the literature.