Density Functional Tight Binding Research Articles

Electronic Nature Transition and Magnetism Creation in Vacancy-Defected Ti2CO2 MXene under Biaxial Strain: A DFTB + U Study.

The structural, electronic, and magnetic properties of vacancy defect in Ti2CO2 MXene and the effect of strain have been investigated using the density functional tight-binding (DFTB) approach including spin-polarization with Hubbard onsite correction (DFTB + U). The band gap of pure Ti2CO2 is ∼1.3 eV, which decreases to ∼0.4 and ∼1.1 eV in the case of C- and O-vacancies, respectively, i.e., the semiconducting behavior is retained. In contrast, Ti2CO2 undergoes semiconductor-to-metal transition by the introduction of a single Ti-vacancy. This transition is the result of introduced localized states in the vicinity of the Fermi level by the vacancy. Both Ti- and O-vacancies have zero net magnetic moments. Interestingly, the nonmagnetic (NM) ground state of semiconducting Ti2CO2 turns into a magnetic semiconductor by introducing a C-vacancy with a magnetization of ∼2 μB/cell. Furthermore, we studied the effect of strain on the electronic structure and magnetic properties of Ti-, C-, and O-vacant Ti2CO2. The nature of the band gap in the presence of single O-vacancy remains indirect in both compression and tensile strain, and the size of the band gap decreases. Compression strain on Ti-vacant Ti2CO2 changes metal into a direct semiconductor, and the metallic character remains under tensile biaxial strain. In opposition, a semiconductor-to-metal transition occurs by applying a compressive biaxial strain on C-vacant Ti2CO2. We also find that the magnetism is preserved under tensile strain and suppressed under compression strain on VC-Ti2CO2. Moreover, we show that double C-vacancies maintain magnetism. Our findings provide important characteristics for the application of the most frequent MXene material and should motivate further investigations because experimentally achieved MXenes always contain point defects.

CO2 adsorption and activation on Ag(1 1 1) surfaces in the presence of surface charge density: A static gas phase DFT study

The CO2 reduction reaction on Ag in the presence of surface plasmons are of strong interest in photocatalysis. Here, two Density Functional Theory (DFT) approaches are proposed to localize charges at the Ag surface to mimic surface plasmon excitation. These calculations predict charge localization at the outermost surface layer, a result confirmed via light excitation at the plasmon resonance modeled by Density Functional Tight Binding Hamiltonian (DFTB) theory. The CO2(gas) reduction initial steps are studied by DFT models showing that bonded species can be created on top of an Ag-atom in the presence of extra charges. A second CO2 molecule can assist the first molecule, decreasing the charge carrier density requirement for CO2 reduction bonded species. A Molecular Dynamics (MD) study shows a possible interaction among CO2 molecule. These results show that a static DFT simulation can mimic charge localization resulting from surface plasmon effects, thus enabling studies on surface plasmon-enhanced chemical reactions, paving the way for future time-dependent (TD) studies.

Obtaining Electronic Properties of Molecules through Combining Density Functional Tight Binding with Machine Learning.

We have introduced a machine learning workflow that allows for optimizing electronic properties in the density functional tight binding method (DFTB). The workflow allows for the optimization of electronic properties by generating two-center integrals, either by training basis function parameters directly or by training a spline model for the diatomic integrals, which are then used to build the Hamiltonian and the overlap matrices. Using our workflow, we have managed to obtain improved electronic properties, such as charge distributions, dipole moments, and approximated polarizabilities. While both machine learning approaches enabled us to improve on the electronic properties of the molecules as compared with existing DFTB parametrizations, only by training on the basis function parameters we were able to obtain consistent Hamiltonians and overlap matrices in the physically reasonable ranges or to improve on multiple electronic properties simultaneously.

Importance of Nuclear Quantum Effects on Aqueous Electrolyte Transport under Confinement in Ti3C2 MXenes.

Protons display a high chemical activity and strongly affect the charge storage capability in confined interlayer spaces of two-dimensional (2D) materials. As such, an accurate representation of proton dynamics under confinement is important for understanding and predicting charge storage dynamics in these materials. While often ignored in atomistic-scale simulations, nuclear quantum effects (NQEs), e.g., tunneling, can be significant under confinement even at room temperature. Using the thermostatted ring polymer molecular dynamics implementation of path integral molecular dynamics (PIMD) in conjunction with the ReaxFF force field, density functional tight binding (DFTB), and NequIP neural network potential simulations, we investigate the role of NQEs on proton and water transport in bulk water and aqueous electrolytes under confinement in Ti3C2 MXenes. Although overall NQEs are relatively small, especially in bulk, we find that they can alter both quantitative values and qualitative trends on both proton transport and water self-diffusion under confinement relative to classical MD predictions. Therefore, our results suggest the need for NQEs to be considered to simulate aqueous systems under confinement for both qualitative and quantitative accuracy.

Exploring the feasibility of new eco-friendly heterocyclic compounds for establishing efficient corrosion protection for N80 steel in a simulated oil well acidizing environment: From molecular-level prediction to experimental validation

Corrosion inhibitors are an effective and low-cost option to relieve strong acid-induced corrosion during acidizing oil wells. In this work, a computational study was conducted to assess the interaction of newly synthesized eco-friendly compounds namely, (R, E)-N′-(4-(dimethylamino)benzylidene)− 2-(4-isobutylphenyl) propanehydrazide (DBIP) and (R,E)− 2-(4-isobutylphenyl)-N′-(4-nitrobenzylidene) propanehydrazide (INBP) with the steel surface. Theoretical prediction of compounds’ affinity to the steel surface and interaction mechanisms were elucidated at the molecular level using first-principles density-functional tight-binding (DFTB), quantum chemical descriptors (QCDs), Fukui functions (FFs) and molecular dynamics (MD) simulations. Then, the corrosion inhibition performance of selected compounds for N80 steel in 15 wt% HCl solution was investigated using weight loss (WL), electrochemical impedance spectroscopy (EIS), potentiodynamic polarization curves (PPCs) methods, and field emission scanning electron microscopy (FESEM). A theoretical investigation by DFTB modeling and Partial Density of States (PDOSs) showed the formation of several covalent bonds between inhibitor molecules and Fe(110) surface. Experimental studies revealed an increased inhibition efficiency of both inhibitors DBIP and INBP with increasing concentrations reaching a higher efficiency of 99% at 5 × 10−3 mol/L of DBIP. The PPCs data confirmed the mixed-type nature of inhibition. The adsorption obeyed the Langmuir isotherm model, following a mixed physical and chemical interaction mode. The surface morphology analysis by FESEM showed a protected N80 steel surface in the presence of inhibitors compared to its highly corroded surface in blank solutions. The present results open an interesting approach for developing a highly efficient corrosion inhibitor formulation for oil well acidizing based on low-cost computational prediction.

The Effect Of Sulphur (S) Doping and K+ Adsorption To The Electronic Properties Of Graphene: A Study By DFTB Method

A study on the effect of S doping and K+ adsorption to the electronic properties of graphene has been conducted by DFTB (Density Functional Tight Binding) calculation. The supercell of 40 x 40 x 1 configured from the 4x4x1 unit cell of graphene was optimized. The calculation shows that the Fermi level of graphene shifted from -4.67 eV into -3.57 eV after S doping. In addition, the S presence caused the formation of gap within the Dirac K of valence band and conduction band. Meanwhile, K+ charge distribution was dominantly occurred within the S-graphene than the graphene.

Performance and interaction mechanism of a new highly efficient benzimidazole-based epoxy resin for corrosion inhibition of carbon steel in HCl: A study based on experimental and first-principles DFTB simulations

Thanks to their excellent properties, epoxy resins are the first choice for many engineering applications. In this regard, herein, a new epoxy resin, namely 1-(oxiran-2-ylmethyl-1H-benzo[d]imidazole (BIZ epoxy), was synthesized and characterized by nuclear magnetic resonance (1H, 13C NMR) and Fourier transform infrared-attenuated total reflection (FTIR-ATR)) techniques. The corrosion inhibition properties of prepared epoxy resin for carbon steel (CS) in 1.0 mol/L HCl were evaluated using electrochemical and scanning electron microscope (SEM) techniques. Furthermore, reactivity and interaction mechanisms between epoxy resin molecule and the iron surface were investigated using quantum chemical parameters (QCPs) and Density-Functional Tight-Binding (DFTB). Electrochemical studies by potentiodynamic polarization curves (PPC) and electrochemical impedance spectroscopy (EIS) showed that the BIZ epoxy acted as a barrier against the acid corrosion of CS with an inhibition efficiency of 93 % at 1 mmol/L. The adsorption of BIZ epoxy followed the Langmuir adsorption isotherm and was found to be a physicochemical process. The SEM analysis showed a significant difference in surface morphology of CS after inhibition due to the formation of a protective barrier. DFTB simulation of BIZ epoxy – Fe(110) surface revealed the formation of a chemical bond and several long-range van der Waals interactions. The projected density of states (PDOS) of isolated and adsorbed systems confirmed the physicochemical nature of interactions between inhibitor molecule and Fe(110) surface.

Understanding interactions between graphene and local anesthetic molecules applied in dentistry – Towards the prolonged effects of local anesthesia

Recent findings have shown that the controlled release of anesthetic molecules via adsorption by nanomaterials is a promising approach to prolonging the effects of local anesthesia. In this work, we have addressed the interactions between graphene and the best-known dental anesthetic molecules: novocaine, lidocaine, and articaine, intending to contribute to developing drug-carrier agents suitable for pain management. A computational study employing various atomistic calculations has been performed to understand in detail how typical representatives of dental anesthetic molecules interact with graphene. The combination of quantum mechanical calculations and molecular dynamics simulations was applied to identify and quantify the non-covalent interactions between graphene and selected anesthetic molecules. Periodic density-functional tight-binding (DFTB) calculations were used to obtain ground state geometries of the system consisting of graphene and adsorbing molecule and calculate the binding energies. Density functional theory (DFT) calculations were used to identify and quantify noncovalent interactions between graphene and adsorbing molecules. Last but not least, molecular dynamics simulations were performed to investigate interactions in water medium, which is important for biomedical applications. It was found that the most prominent prolongation of the anesthetic effect via adsorption by graphene could be potentially achieved in the case of articaine.

Efficient simulation of resonance Raman spectra with tight-binding approximations to density functional theory.

Resonance Raman spectroscopy has long been established as one of the most sensitive techniques for detection, structure characterization, and probing the excited-state dynamics of biochemical systems. However, the analysis of resonance Raman spectra is much facilitated when measurements are accompanied by Density Functional Theory (DFT) calculations that are expensive for large biomolecules. In this work, resonance Raman spectra are therefore computed with the Density Functional Tight-Binding (DFTB) method in the time-dependent excited-state gradient approximation. To test the accuracy of the tight-binding approximations, this method is first applied to typical resonance Raman benchmark molecules, such as β-carotene, and compared to results obtained with pure and range-separated exchange-correlation functionals. We then demonstrate the efficiency of the approach by considering a computationally challenging heme variation. Overall, we find that the vibrational frequencies and excited-state properties (energies and gradients) that are needed to simulate the spectra are reasonably accurate and suitable for interpretation of experiments. We can therefore recommend DFTB as a fast computational method to interpret resonance Raman spectra.

Direct Electrochemical CO2 Capture Using Substituted Anthraquinones in Homogeneous Solutions: A Joint Experimental and Theoretical Study.

Electrochemical capture of carbon dioxide (CO2) using organic quinones is a promising and intensively studied alternative to the industrially established scrubbing processes. While recent studies focused only on the influence of substituents having a simple mesomeric or nucleophilicity effect, we have systematically selected six anthraquinone (AQ) derivatives (X-AQ) with amino and hydroxy substituents in order to thoroughly study the influence thereof on the properties of electrochemical CO2 capture. Experimental data from cyclic voltammetry (CV) and UV–Vis spectroelectrochemistry of solutions in acetonitrile were analyzed and compared with innovative density functional tight binding computational results. Our experimental and theoretical results provide a coherent explanation of the influence of CO2 on the CV data in terms of weak and strong binding nomenclature of the dianions. In addition to this terminology, we have identified the dihydroxy substituted AQ as a new class of molecules forming rather unstable [X-AQ-(CO2)n]2– adducts. In contrast to the commonly used dianion consideration, the results presented herein reveal opposite trends in stability for the X-AQ-CO2•– radical species for the first time. To the best of our knowledge, this study presents theoretically calculated UV–Vis spectra for the various CO2-AQ reduction products for the first time, enabling a detailed decomposition of the spectroelectrochemical data. Thus, this work provides an extension of the existing classification with proof of the existence of X-AQ-CO2 species, which will be the basis of future studies focusing on improved materials for electrochemical CO2 capture.

Design of new hole transport materials based on triphenylamine derivatives using different π-linkers for the application in perovskite solar cells. A theoretical study.

New organic molecules containing five different compounds, commonly called p-linkers, located between the triphenylamine units, were theoretically designed and analyzed in order to be proposed as new hole transport materials (HTMs) in perovskite solar cells, in total ten new molecules were analyzed. The electronic, optical and hole transport properties were determined, similarly, the relationship of these properties with their molecular structure was also investigated by Density Functional Theory (DFT) and Density Functional Tight Binding (DFTB) calculations. Eight of the ten analyzed compounds exhibited the main absorption band out of the visible region; therefore these compounds did not present an overlap with the absorption spectra of the typical methylammonium lead iodide (MAPI) hybrid-perovskite. The results showed that the Highest occupied molecular orbital (HOMO) levels of the compounds are higher than the perovskite HOMO level, and in some cases these are even higher than the Spiro-OMeTAD HOMO. The calculated electronic couplings and the reorganization energy values provided useful information in order to determine if the systems were hole or electron transport materials.

Interaction of CO2 with TiO2/reduced graphene oxide as superior catalysts: Dispersion-corrected density functional theory simulation

Interaction of CO2 with the anatase TiO2 slab, nano-TiO2 cluster and nano-TiO2/reduced graphene oxide (rGO) was explored by density functional theory simulations. Two orientations, namely (001) and (100) directions, were examined for an approaching CO2 molecule to the surfaces. The energetically favorable state corresponds to a CO2 molecule adsorbed in the (001) direction for all the considered substrates. The interaction energies of adsorbed CO2 on the TiO2 slab, nano-TiO2 cluster, and nano-TiO2-rGO (on the nano-TiO2 cluster attached to the rGO) surfaces were determined to be about −5.007, −11.643, and −13.025 kcal/mol, respectively. Interaction properties were identified by the high-level MP2 calculations and the AIM theory analysis. Finally, for the adsorbed CO2 on the nano-TiO2-rGO, density-functional tight-binding (DFTB) molecular dynamics simulation were carried out and the results showed that CO2 absorption strength was enhanced on the (001) nano-TiO2 at room temperature by bonding of either O atom or C atom to the surface Ti and O atoms. It is found that nano-TiO2-rGO was more effective for CO2 adsorption than both the bulk TiO2 slab and free nano-TiO2 cluster.

A quantum chemical molecular dynamics repository of solvated ions

The importance of ion-solvent interactions in predicting specific ion effects in contexts ranging from viral activity through to electrolyte viscosity cannot be underestimated. Moreover, investigations of specific ion effects in nonaqueous systems, highly relevant to battery technologies, biochemical systems and colloid science, are severely limited by data deficiency. Here, we report IonSolvR – a collection of more than 3,000 distinct nanosecond-scale ab initio molecular dynamics simulations of ions in aqueous and non-aqueous solvent environments at varying effective concentrations. Density functional tight binding (DFTB) is used to detail the solvation structure of up to 55 solutes in 28 different protic and aprotic solvents. DFTB is a fast quantum chemical method, and as such enables us to bridge the gap between efficient computational scaling and maintaining accuracy, while using an internally-consistent simulation technique. We validate the database against experimental data and provide guidance for accessing individual IonSolvR records.

Intrinsic electronic property and adsorption of organic molecules on specific iron surface: an ab initio DFT and DFTB study

An ab initio density functional theory (DFT) calculation revealed the electronic properties of azomethine functionalised epoxy, namely, 4-((oxiran-2-yl)methoxy)-N-(4-(4-(4-((oxiran-2-yl)methoxy)benzylideneamino)phenoxy)benzylidene)benzenamine (DSBE) and amine based curing agent namely diethylenetriamine (DETA), triethylenetetraamine (TETA) and para-phenylenediamine (PPD) responsible for its adsorption on the mild steel surface. The frontiers molecular orbitals (FMOs) and the corresponding energies as well as global softness values revealed the reactive nature of the studied organic molecules. The probable trends of the chemical properties of different amine cured epoxy have been predicted. The insights of the adsorption of these organic molecules have been visualized through density functional tight binding calculation. It revealed that these organic molecules are able to adsorb onto targeted metal surface viz. Fe(110), FeO(110) and Fe2O3(110) plane through charge density sharing occurring at the molecule-iron layer interface. Thus, a complete theoretical insight analysis and modelled simulated adsorption study provided a clear picture regarding the adsorptions of the organic molecules onto metallic surfaces.

(Invited) Simulating Dynamic Excitons Via Quantum Molecular Dynamics: A Case Study in Lead Halide Perovskites

The ultrafast electronic and structural dynamics invoked by photoexcitation, i.e., “dynamic exciton” phenomena, not only are important in the context of practical applications such as solar cells, but also raise many questions from the viewpoint of fundamental science. Experimental measurement, computational simulations, and theoretical interpretation will be the three pillars for deciphering the dynamic exciton phenomena.From the viewpoint of computational simulations, molecular dynamics (MD) techniques combined with quantum chemical calculations, i.e., quantum molecular dynamics (QMD), has been the popular tool to simulate the dynamic exciton phenomena. The quantum chemical calculations, which are typically conducted on the basis of the density-functional theory (DFT), require the large computational resources and time. These have been the limiting factors for the accessible spatial and time scales by the QMD simulations.To extend the coverage of simulations to more complex, large-scale systems, we have developed efficient excited-state QMD methods that can include nonadiabatic effects. Our method combines the density-functional tight binding (DFTB) method, which is an approximate DFT, and the surface hopping method, which is a theoretical framework to incorporate the nonadiabatic effects into the QMD simulations. The method was further improved to be suitable for condensed-phase simulations explicitly including the environment, i.e., solvent, by using a “divide-and-conquer” style quantum chemical calculation technique. These theoretical framework enables us to simulate the coupled electronic–structural dynamics in excited states of systems consisting of 102–103 atoms[1,2,3].In addition, using the developed method, we conducted the real-time simulations of the ultrafast processes invoked by photoexcitation of lead iodide perovskites, which are known as the key materials for perovskite solar cells. The dissociation of the exciton into the positive and negative charge carriers was observed. Moreover, the hot carrier cooling, where the charge carriers dissipate excess energy via the electron–phonon coupling and relax to the band edges, was also tracked. Finally, the direct evidence of the polaron formation, where the structural deformation is induced by the presence of charge carriers, was observed. These results highlight the importance of the coupling between electronic and structural degrees of freedom.In the talk, recent improvements in the methodology and future perspectives will also be presented[5].

Molecular dynamics study of crystal lattice effect on mechanical properties of SiO2: A density functional tight binding investigation

In this study, molecular dynamics simulations (MD) based on quantum mechanical method in which the interactions were expressed by self-consistent charge density functional tight binding (SCC-DFTB) to investigate the mechanical properties of four different SiO2 structures. The Young's, shear and bulk modulus of systems are calculated and the results compared to experimental and other computational findings. We have observed that the hexagonal SiO2 exhibits more different mechanical behavior than other SiO2 structures because of its negative bulk modulus value. While the length of a-lattice parameters for four SiO2 crystal structures increase with the change of MD time, the length of the b and c-lattice vectors shortens. Moreover, it is analyzed the stress-strain behavior and yield points are determined from this curve of SiO2 structures. It is revealed that the plastic deformation occurs at higher stress for hexagonal SiO2 than other SiO2 structures because of its rigid solid property. These results can provide a reference for understanding the mechanical properties of various SiO2 crystal structures.

Atomistic Modeling of the Electrical Conductivity of Single‐Walled Carbon Nanotube Junctions

Carbon nanotubes (CNTs) have many interesting properties that make them a focus of research in a wide range of technological applications. In CNT films, the bottleneck in charge transport is typically attributed to higher resistance at CNT junctions, leading to electrical transport characteristics that are quite different from individual CNTs. Previous simulations confirm this; however, a systematic study of transport across junctions is still lacking in the literature. Herein, density functional tight binding (DFTB) theory combined with the nonequilibrium Green's functions (NEGF) method is used to systematically calculate current across a range of CNT junctions. A random sampling approach is used to sample an extensive library of junction structures. The results demonstrate that the conductivity of CNT contacts depends on the overlap area between nanotubes and exponentially on the distances between the carbon atoms of the interacting CNTs. Two models based solely on the atomic positions of carbon atoms within the nanotubes are developed and evaluated: a simple equation using only the smallest C–C separation and a more sophisticated model using the positions of all C atoms. These junction current models can be used to predict transport in larger‐scale simulations where the CNT fabric structure is known.

Nonadiabatic Molecular Dynamics with Extended Density Functional Tight-Binding: Application to Nanocrystals and Periodic Solids.

In this work, we report a new methodology for nonadiabatic molecular dynamics calculations within the extended tight-binding (xTB) framework. We demonstrate the applicability of the developed approach to finite and periodic systems with thousands of atoms by modeling "hot" electron relaxation dynamics in silicon nanocrystals and electron-hole recombination in both a graphitic carbon nitride monolayer and a titanium-based metal-organic framework (MOF). This work reports the nonadiabatic dynamic simulations in the largest Si nanocrystals studied so far by the xTB framework, with diameters up to 3.5 nm. For silicon nanocrystals, we find a non-monotonic dependence of "hot" electron relaxation rates on the nanocrystal size, in agreement with available experimental reports. We rationalize this relationship by a combination of decreasing nonadiabatic couplings related to system size and the increase of available coherent transfer pathways in systems with higher densities of states. We emphasize the importance of proper treatment of coherences for obtaining such non-monotonic dependences. We characterize the electron-hole recombination dynamics in the graphitic carbon nitride monolayer and the Ti-containing MOF. We demonstrate the importance of spin-adaptation and proper sampling of surface hopping trajectories in modeling such processes. We also assess several trajectory surface hopping schemes and highlight their distinct qualitative behavior in modeling the excited-state dynamics in superexchange-like models depending on how they handle coherences between nearly parallel states.

Effects of chiral index, vacancy defects and external electric field on the structures and electronic properties of DWSiNTs by SCC-DFTB calculations

As a new candidate material, silicon nanotubes (SiNTs) have been widely studied. However, various types of structural defects and external electric field are usually inescapability introduced in the experimental process of SiNTs synthesis. Structures, stability and electronic properties of the materials especially for double-walled silicon nanotubes (DWSiNTs) require to be considered. In this work, effects of vacancy defects and external electric field on properties of the geometrically structural optimization and electronic properties of DWSiNTs zigzag (3,0)@(7,0) and armchair (3,3)@(7,7) are studied via the self-consistent charge density functional tight binding (SCC-DFTB) method. The results of us show that the regular arrangement of the atoms, quantum molecular descriptors, distribution of charge, the degree of warping, stability, energy gap and Fermi energy levels intensely hinge on the chirality index of the tube and impacts of vacancy defects and electric field. Especially, the appearance of monovacancy and the change of vacancy density have the most significant effect on the band gap. For the pristine tubes, the charges transfer from the inside to the outside along the radial direction of the tube exhibiting an axisymmetric distribution. With the introduction of defects, the range of the charge transfer is broadened dramatically. The direction of charge transfer in the applied electric field is always opposite to the field direction. Our work may provide a new route to tune the electronic properties of DWSiNTs and opens a wider field of its application in nano-electronic devices.

A Hybrid Machine Learning Approach for Structure Stability Prediction in Molecular Co-crystal Screenings

Co-crystals are a highly interesting material class as varying their components and stoichiometry in principle allows tuning supramolecular assemblies toward desired physical properties. The in silico prediction of co-crystal structures represents a daunting task, however, as they span a vast search space and usually feature large unit cells. This requires theoretical models that are accurate and fast to evaluate, a combination that can in principle be accomplished by modern machine-learned (ML) potentials trained on first-principles data. Crucially, these ML potentials need to account for the description of long-range interactions, which are essential for the stability and structure of molecular crystals. In this contribution, we present a strategy for developing Δ-ML potentials for co-crystals, which use a physical baseline model to describe long-range interactions. The applicability of this approach is demonstrated for co-crystals of variable composition consisting of an active pharmaceutical ingredient and various co-formers. We find that the Δ-ML approach offers a strong and consistent improvement over the density functional tight binding baseline. Importantly, this even holds true when extrapolating beyond the scope of the training set, for instance in molecular dynamics simulations under ambient conditions.