First-principles Electronic Structure Methods Research Articles

Strain aided drastic reduction in lattice thermal conductivity and improved thermoelectric properties in Janus MXenes.

Surface and strain engineering are among the cheaper ways to modulate structure property relations in materials. Due to their compositional flexibilities, MXenes, the family of two-dimensional materials, provide enough opportunity for surface engineering. In this work, we have explored the possibility of improving thermoelectric efficiency of MXenes through these routes. The Janus MXenes obtained by modifications of the transition metal constituents and the functional groups passivating their surfaces are considered as surface engineered materials on which bi-axial strain is applied in a systematic way. We find that in the three Janus compounds Zr2COS, ZrHfCO2and ZrHfCOS, tensile strain modifies the electronic and lattice thermoelectric parameters such that the thermoelectric efficiency can be maximised. A remarkable reduction in the lattice thermal conductivity due to increased anharmonicity and elevation in Seebeck coefficient are obtained by application of moderate tensile strain. With the help of first-principles electronic structure method and semi-classical Boltzmann transport theory we analyse the interplay of structural parameters, electronic and dynamical properties to understand the effects of strain and surface modifications on thermoelectric properties of these systems. Our detailed calculations and in depth analysis lead not only to the microscopic understanding of the influences of surface and strain engineering in these three systems, but also provide enough insights for adopting this approach and improve thermoelectric efficiencies in similar systems.

Impact of Static Distortion Waves on Superlubricity.

Friction is a major source of energy loss in mechanical devices. This energy loss may be minimized by creating interfaces with extremely reduced friction, i.e., superlubricity. Conventional wisdom holds that incommensurate interface structures facilitate superlubricity. Accurately describing friction necessitates the precise modeling of the interface structure. This, in turn, requires the use of accurate first-principles electronic structure methods, especially when studying organic/metal interfaces, which are highly relevant due to their tunability and propensity to form incommensurate structures. However, the system size required to calculate incommensurate structures renders such calculations intractable. As a result, studies of incommensurate interfaces have been limited to very simple model systems or strongly simplified methodology. We overcome this limitation by developing a machine-learned interatomic potential that is able to determine energies and forces for structures containing thousands to tens of thousands of atoms with an accuracy comparable to conventional first-principles methods but at a fraction of the cost. Using this approach, we quantify the breakdown of superlubricity in incommensurate structures due to the formation of static distortion waves. Moreover, we extract design principles to engineer incommensurate interface systems where the formation of static distortion waves is suppressed, which facilitates low friction coefficients.

Estimating Free Energy Barriers for Heterogeneous Catalytic Reactions with Machine Learning Potentials and Umbrella Integration.

Predicting the rate constants of elementary reaction steps is key for the computational modeling of catalytic processes. Within transition state theory (TST), this requires an accurate estimation of the corresponding free energy barriers. While sophisticated methods for estimating free energy differences exist, these typically require extensive (biased) molecular dynamics simulations that are computationally prohibitive with the first-principles electronic structure methods that are typically used in catalysis research. In this contribution, we show that machine-learning (ML) interatomic potentials can be trained in an automated iterative workflow to perform such free energy calculations at a much reduced computational cost as compared to a direct density functional theory (DFT) based evaluation. For the decomposition of CHO on Rh(111), we find that thermal effects are substantial and lead to a decrease in the free energy barrier, which can be vanishingly small, depending on the DFT functional used. This is in stark contrast to previously reported estimates based on a harmonic TST approximation, which predicted an increase in the barrier at elevated temperatures. Since CHO is the reactant of the putative rate limiting reaction step in syngas conversion on Rh(111) and essential for the selectivity toward oxygenates containing multiple carbon atoms (C2+ oxygenates), our results call into question the reported mechanism established by microkinetic models.

Semi-empirical Haken–Strobl model for molecular spin qubits

Understanding the physical processes that determine the relaxation T 1 and dephasing T 2 times of molecular spin qubits is critical for envisioned applications in quantum metrology and information processing. Recent spin-echo measurements of solid-state molecular spin qubits have stimulated the development of quantum mechanical models for predicting intrinsic qubit timescales using first-principles electronic structure methods. We develop an alternative semi-empirical approach to construct Redfield quantum master equations for molecular spin qubits using a stochastic Haken–Strobl theory for a central spin with fluctuating gyromagnetic tensor due to spin-lattice interaction and fluctuating local magnetic field due to interactions with lattice spins. Using two vanadium-based spin qubits as case studies, we compute qubit population and decoherence times as a function of temperature and magnetic field, using a bath spectral density parametrized with a small number of T 1 measurements. The theory quantitatively agrees with experimental data over a range of conditions beyond those used to parameterize the model, demonstrating the generalization potential of the method. The ability of the model to describe the temperature dependence of the ratio is discussed and possible applications for designing novel molecule-based quantum magnetometers are suggested.

Structural arrangement and improved thermoelectric figure of merit in hexagonal SiX (X = N, P, As, Sb, Bi) monolayers: understanding from first-principles calculations

Materials with thermoelectric (TE) efficiencies comparable to Carnot efficiency are desirable for applications in devices that use TE effects to produce electricity from heat. The figure of merit of a TE material is a measure of its efficiency. This quantity is determined by the intricate interplay of electronic and thermal transport parameters. Layered materials at low dimensions, such as 2D monolayers, have been in focus with regard to the quest for new TE materials. Group III–VI semiconductors in their 2D monolayers have exhibited fascinating TE properties. Naturally, it is expected that isoelectronic IV–V semiconductors will possess similar novelty. In this work, we have investigated the electronic, dynamical and TE properties of IV–V monolayer SiN, SiP, SiAs, SiSb and SiBi using first-principles electronic structure methods in conjunction with the semiclassical Boltzmann transport theory. Unlike previous studies, here the focus was to compute and analyse the effects of structural arrangements on TE properties. We found that upon consideration of an alternative stacking arrangement in the systems, significant changes occur in the transport phenomena, leading to a large figure of merit and thus TE efficiency. A comprehensive comparative analysis is presented to interpret the results. This work bears significance in the fact that it demonstrates a very important structure–property relationship aspect for these compounds that has been overlooked so far, and this approach may lead to the discovery of new TE materials with desirable efficiencies.

Ferroelectric HfO2 and the importance of strain

Ferroelectric oxides based on ${\mathrm{HfO}}_{2}$ show tremendous promise for the next generation of memory and logic devices. The ferroelectric polymorph is one of several that can be derived from the high symmetry cubic fluorite structure of ${\mathrm{HfO}}_{2}$. A single grain of ${\mathrm{HfO}}_{2}$ may consist of a coherent mixture of multiple orientational and translational variants of different polymorphs. Here, we use symmetry-adapted strain-order parameters to elucidate the relationship between the different ${\mathrm{HfO}}_{2}$ polymorphs and their symmetrically equivalent variants. We use first-principles electronic structure methods to identify minimum energy pathways and map them in subspaces of the symmetry-adapted strain order parameters. We next investigate the atomic structure of domain boundaries that separate coexisting variants of ferroelectric ${\mathrm{HfO}}_{2}$. We rely on Gibbsian excess quantities and a precise specification of mechanical boundary conditions to describe the thermodynamic properties of domain boundaries. Our first-principles calculations show that the O and Hf shuffle arrangement within a domain boundary is closely related to the intermediate shuffle patterns of the homogeneous pathways between ferroelectric variants. Furthermore, the preferred structure within a boundary is very sensitive to local strain constraints imposed by the adjacent ferroelectric variants, leading to highly anisotropic domain boundary energies.

CENT2: Improved charge equilibration via neural network technique

The high computational cost of first-principles electronic structure methods together with the successful applications of machine learning (ML) techniques in atomistic simulations resulted in a surge of interest in ML-based interatomic potentials. Despite great progress in the field, there remain some challenges to be solved such as the best way of incorporating long-range interactions, as well as nonlocal charge transfer. The first generation of the charge equilibration via neural network technique (CENT) was a major step forward in concurrently taking into account both aforementioned points. Within structure prediction methods, it turned out to be a powerful tool in discovering novel polymorphs of ionic systems. On the other hand, the method is not expected to be appropriate for multicomponent systems with reference data sets in which some or all elements are subject to varying oxidation states. Here, we present the second generation of CENT, with multiple improvements to the original variant that lead to a more accurate treatment of electrostatic interactions. To do this, it aims at reproducing the electric potential function, which is directly related to the charge distribution, rather than only considering total energies. In addition, a charge-free term is added to correct for the difference between the reference energies and those obtained with the energy functional of CENT. Moreover, the Green's function within the Hartree energy is modified to substantially shield interactions from charges in the neighborhood of each point. Also, the charge density is split into ionic and electronic parts, which allows for a better approximation of the electron density. The utility of this method is examined for magnesium oxide clusters, and multiple comparisons with the first generation are made, demonstrating that much more physical electrostatic interactions can be expected from the second generation of CENT.

Ab initio metadynamics determination of temperature-dependent free-energy landscape in ultrasmall silver clusters.

Ab initio metadynamics enables the extraction of free-energy landscapes having the accuracy of first-principles electronic structure methods. We introduce an interface between the PLUMED code that computes free-energy landscapes and enhanced-sampling algorithms and the Atomic Simulation Environment (ASE) module, which includes several ab initio electronic structure codes. The interface is validated with a Lennard-Jones cluster free-energy landscape calculation by averaging multiple short metadynamics trajectories. We use this interface and analysis to estimate the free-energy landscape of Ag5 and Ag6 clusters at 10, 100, and 300K with the radius of gyration and coordination number as collective variables, finding at most tens of meV in error. Relative free-energy differences between the planar and non-planar isomers of both clusters decrease with temperature in agreement with previously proposed stabilization of non-planar isomers. Interestingly, we find that Ag6 is the smallest silver cluster where entropic effects at room temperature boost the non-planar isomer probability to a competing state. The new ASE-PLUMED interface enables simulating nanosystem electronic properties under more realistic temperature-dependent conditions.

Microscopic origin of multiferroic order in monolayer NiI2

The discovery of multiferroic behavior in monolayer NiI2 provides a new symmetry-broken state in van der Waals monolayers, featuring the simultaneous emergence of helimagnetic order and ferroelectric order at a critical temperature of T = 21 K. However, the microscopic origin of multiferroic order in NiI2 monolayer has not been established, and in particular, the role of non-collinear magnetism and spin–orbit coupling in this compound remains an open problem. Here we reveal the origin of the two-dimensional multiferroicity in NiI2 using first-principles electronic structure methods. We show that the helimagnetic state appears as a consequence of the long-range magnetic exchange interactions, featuring sizable magnetic moments in the iodine atoms. We demonstrate that the electronic density reconstruction accounting for the ferroelectric order emerges from the interplay of non-collinear magnetism and spin–orbit coupling. We demonstrate that the ferroelectric order is controlled by the iodine spin–orbit coupling, and leads to an associated electronically-driven distortion in the lattice. Our results establish the microscopic origin of the multiferroic behavior in monolayer NiI2, putting forward the coexistence of helical magnetic order and ligand spin–orbit coupling as driving forces for multiferroic behavior in two-dimensional materials.

Anomalous Hall signatures of nonsymmorphic nodal lines in the doped chromium chalcospinel CuCr2Se4

An emerging phase of matter among the class of topological materials is nodal line semimetal, possessing symmetry-protected one-dimensional gapless lines at the (or close to) the Fermi level in $k$-space. When the $k$-dispersion of the nodal line is weak, van Hove singularities generated by the almost flat nodal lines may be prone to instabilities introduced by additional perturbations such as spin-orbit coupling or magnetism. Here, we study Cr-based ferromagnetic chalcospinel compound CuCr$_2$Se$_4$ (CCS) via first-principles electronic structure methods and reveal the true origin of its dissipationless anomalous Hall conductivity, which was not well understood previously. We find that CCS hosts nodal lines protected by nonsymmorphic symmetries, located in the vicinity of Fermi level, and that such nodal lines are the origin of the previously observed distinct behavior of the anomalous Hall signature in the presence of electron doping. The splitting of nodal line via spin-orbit coupling produces a large Berry curvature, which leads to a significant response in anomalous Hall conductivity. Upon electron doping via chemical substitution or gating, or rotation of magnetization via external magnetic field, steep change of anomalous Hall behavior occurs, which makes CCS a promising compound for low energy spintronics applications.

Theoretical investigation of capacitances in functionalised MXene supercapacitors M n+1C n O2, M = Ti, V, Nb, Mo

MXene, the class of two-dimensional materials, has been found to be useful as potential electrode materials for electrochemical capacitors. Although experimental investigation on the electrochemical performances of a few MXenes have been carried out with exciting results, a complete understanding of their atomic scale behaviour is yet to be done. Using first-principles electronic structure methods, we perform a systematic investigation of the capacitances in pristine and functionalised MXenes where M = Ti, V, Nb and Mo. We provide results on each of the three sources of the capacitance and analyse them in detail for a complete understanding of their behaviour. The interpretation of the experimental results, wherever available, in the light of our computations, provides useful insights.

Kerker mixing scheme for self-consistent muffin-tin based all-electron electronic structure calculations

We propose a computationally efficient Kerker mixing scheme for robust and rapidly converging self-consistent-field calculations using all-electron first-principles electronic structure methods based on the muffin-tin partitioning of space. The mixing scheme is composed of the Kerker preconditioner in combination with quasi-Newton methods. We construct the Kerker preconditioner in the muffin-tin sphere by determining the screened Coulomb potential in real space, solving a modified Helmholtz equation by adopting Weinert's pseudocharge method for calculating the Poisson equation for periodic charge densities without shape approximation to the solution of the modified Helmholtz equation. Implemented in a full-potential linearized augmented plane-wave (FLAPW) method, we found that the Kerker preconditioning scheme (i) leads to a convergence to self-consistency that is independent of system size, (ii) is extremely robust in the choice of the mixing and preconditioning parameters, (iii) scales linearly with system size in computational cost, and (iv) conserves the total charge. We have related the preconditioning parameter to the density of states of the delocalized electrons at the Fermi energy and developed a model to choose the preconditioning parameter either prior to the calculation or on the fly. Our computationally validated model supports the hypothesis that, in the absence of Kerker preconditioning, the delocalized $s$ and $p$ electrons of simple and transition metals are the primary cause for the slowing of the convergence speed and that the stronger, localized $d$ and $f$ electrons account for only a small fraction of the charge sloshing problem. The presented formulation of the Kerker preconditioning scheme establishes an efficient methodology for the simulation of magnetic and nonmagnetic metallic large-scale material systems by means of muffin-tin-based all-electron methods.

Protein Molecular Dynamics Simulations with Approximate QM: What Can We Learn?

Classical force fields are essential for computer simulations of proteins and are typically parameterized to reproduce secondary and tertiary structure of isolated proteins. However, while protein-protein interactions are ubiquitous in nature, they are not considered in parameterization efforts and are far less understood than isolated proteins. A better characterization of intermolecular interactions is widely recognized as a key to revolutionizing drug and therapeutic developments with high-throughput computational screening. Urgently needed is a critical assessment of the performance of modern protein force fields against first-principles electronic structure methods and experiments. In a daring step toward this goal, we here describe a comparison of peptide folding dynamics as predicted by a molecular mechanics force field on the one hand and by an approximate electronic structure quantum mechanical (QM) method based on density-functional tight-binding (DFTB) on the other. We further compare the dynamics from straightforward DFTB simulations with a near-linear scaling version of DFTB for massively parallel computation based on the fragment molecular orbital (FMO-DFTB) method. We illustrate differences between the phenomenology of the folding dynamics from these three methods for a small model peptide, as well as charge polarization and dynamic fluctuations, point out possible correlations and implications for force field developers, and discuss the lessons learned that might become applicable to future predictive high-throughput computer screening for personalized neoantigen cancer therapy.

Enhancement of the electronic thermoelectric properties of bulk strained silicon-germanium alloys using the scattering relaxation times from first-principles calculations

We use first-principles electronic structure methods to calculate the electronic thermoelectric properties (i.e., due to electronic transport only) of single-crystalline bulk n-type silicon-germanium alloys vs Ge composition, temperature, doping concentration, and strain. We find excellent agreement to available experiments for the resistivity, mobility, and Seebeck coefficient. These results are combined with the experimental lattice thermal conductivity to calculate the thermoelectric figure of merit ZT, finding very good agreement with experiments. We predict that 3% tensile hydrostatic strain enhances the n-type ZT by 50% at carrier concentrations of n=1020cm−3 and a temperature of T=1200K. These enhancements occur at different alloy compositions due to different effects: at 50% Ge composition, the enhancements are achieved by a strain induced decrease in the Lorenz number, while the power factor remains unchanged. These characteristics are important for highly doped and high temperature materials, in which up to 50% of the heat is carried by electrons. At 70% Ge, the increase in ZT is due to a large increase in the electrical conductivity produced by populating the high mobility Γ conduction band valley, lowered in energy by strain.

High thermoelectric properties in full-Heusler X2YZ alloys (X = Ca, Sr, and Ba; Y = Au and Hg; Z = Sn, Pb, As, Sb, and Bi)

The electronic and transport properties of a new class of full Heusler semiconductors X2YZ (X = Ca, Sr, and Ba; Y = Au and Hg; Z = As, Sb, Bi, Sn and Pb) are investigated using first-principles electronic structure methods and Boltzmann transport theory. The results show that the spin–orbit coupling not only affects the band-gap values but also the symmetry and dispersion of the top of the valence and bottom of the conduction band, so it should not be neglected in computation of electron transport properties. The electronegativity significantly affects the energy band gap and the strength of the Y-Z covalent bond, and thus affects the transport properties. By comparison the electronic transport properties of X2YZ with that of Fe2VAl and taking the lattice thermal conductivity into account, we predict the minimum value of the optimal figure of merit (ZT) should be more than 2.0 at 300 K. Interestingly, for the same carrier concentration, very similar p- and n-type thermoelectric performance is observed in both Ba2AuBi and Ba2AuSb, which is highly important for the fabrication of the thermoelectric modules with comparable efficiencies.

Magnetic exchange interactions and phase transition in Fe/V(100), V/Fe/V(100) and Fe/V/Fe/V(100) films

We present the magnetic properties of bcc Fe films on and embedded in bcc-V(100) substrates in their paramagnetic phase above the Curie temperature. We have used a first-principles electronic structure method based on the spin-density functional theory and the spin fluctuations in the paramagnetic phase are described using the disordered local moments model. We have calculated the local magnetic moments, magnetic correlations and the Curie temperature of several Fe/V(100) and V/Fe/V(100) systems. In general, the Fe layers in close proximity to V have their magnetic moments suppressed, while in the interior of the film, the value of the magnetic moment oscillates from one layer to the other. When the films are capped by a V layer, the magnitudes of the magnetic moments decreases. The Curie temperature also shows an interesting oscullatory behaviour as the V capping layer thickness is increased. In the Fe/V/Fe/V(100) films, we observed that the exchange interactions between the two Fe layers across the spacer exhibits an oscillating behaviour.

Structure Dependent Phase Stability and Thermal Expansion of Ruddlesden–Popper Strontium Titanates

Ruddlesden–Popper strontium titanates with composition Srn+1TinO3n+1, RP-n-STO, n = 1, 2, ..., ∞, are prototypical layered perovskites from which many crystal-chemistry principles can be assessed. In addition, their phase stabilities and lattice thermal expansions are essential factors for their usage in realistic devices. In this work, we use first-principles electronic structure methods to determine the n-layer dependent phase stabilities, including thermodynamic energies and pressure–critical temperature (P–Tc) phase diagrams and coefficients of thermal expansion. We correlate these properties with the perovskite (SrTiO3)n·SrO block thickness, n, to show that a change in materials behavior occurs at a critical thickness nc = 3, which we ascribe to the presence (absence) of octahedral layers exhibiting antiferrodistortions at n ≥ 3 (n < 3). Last, we attribute the absence of quasi-two-dimensional lattice anharmonicity in the layered RP titanates to the stiff (SrTiO3)n intrablock Ti–O bonds and strong int...

First-Principles Models for Biological Light-Harvesting: Phycobiliprotein Complexes from Cryptophyte Algae.

There have been numerous efforts, both experimental and theoretical, that have attempted to parametrize model Hamiltonians to describe excited state energy transfer in photosynthetic light harvesting systems. The Frenkel exciton model, with its set of electronically coupled two level chromophores that are each linearly coupled to dissipative baths of harmonic oscillators, has become the workhorse of this field. The challenges to parametrizing such Hamiltonians have been their uniqueness, and physical interpretation. Here we present a computational approach that uses accurate first-principles electronic structure methods to compute unique model parameters for a collection of local minima that are sampled with molecular dynamics and QM geometry optimization enabling the construction of an ensemble of local models that captures fluctuations as these systems move between local basins of inherent structure. The accuracy, robustness, and reliability of the approach is demonstrated in an application to the phycobiliprotein light harvesting complexes from cryptophyte algae. Our computed Hamiltonian ensemble provides a first-principles description of inhomogeneous broadening processes, and a standard approximate non-Markovian reduced density matrix dynamics description is used to estimate lifetime broadening contributions to the spectral line shape arising from electronic-vibrational coupling. Despite the overbroadening arising from this approximate line shape theory, we demonstrate that our model Hamiltonian ensemble approach is able to provide a reliable fully first-principles method for computation of spectra and can distinguish the influence of different chromophore protonation states in experimental results. A key feature in the dynamics of these systems is the excitation of intrachromophore vibrations upon electronic excitation and energy transfer. We demonstrate that the Hamiltonian ensemble approach provides a reliable first-principles description of these contributions that have been detailed in recent broad-band pump-probe and two-dimensional electronic spectroscopy experiments.

Controlling the Conductance of a Graphene-Molecule Nanojunction by Proton Transfer.

The possibility of using single molecule junctions as components of nanoelectronic devices has motivated intensive experimental and theoretical research on the underlying transport mechanism in these systems. In this Letter, we investigate from a theoretical perspective intramolecular proton transfer reactions as a mechanism for controlling the conductance state of graphene-based molecular junctions. Employing a methodology that combines first-principles electronic structure methods with transport approaches, we show that the proton transfer reaction proceeds via a stepwise mechanism, giving rise to several tautomers with different conductance states. The analysis reveals that the relative stability of the tautomers as well as the energy barrier for their interconversion can be controlled by means of an external electrostatic field, which provides a mechanism for switching the nanojunction.

Elastic and piezoresistive properties of nickel carbides from first principles

The nickel--carbon system has received increased attention over the past years due to the relevance of nickel as a catalyst for carbon nanotube and graphene growth, where Nickel carbide intermediates may be involved or carbide interface layers form in the end. Nickel--carbon composite thin films comprising Ni$_3$C are especially interesting in mechanical sensing applications. Due to the meta-stability of nickel carbides, formation conditions and the coupling between mechanical and electrical properties are not yet well understood. Using first-principles electronic structure methods, we calculated the elastic properties of Ni$_3$C, Ni$_2$C and NiC, as well as changes in electronic properties under mechanical strain. We observe that the electronic density of states around the Fermi level does not change under the considered strains of up to 1%, which correspond to stresses up to 3GPa. Relative changes in conductivity of Ni$_3$C range up to maximum values of about 10%.