LCAO Research Articles

LCAO Electronic Structure of Nucleic Acid Bases and Other Heterocycles and Transfer Integrals in B-DNA, Including Structural Variability.

To describe the molecular electronic structure of nucleic acid bases and other heterocycles, we employ the Linear Combination of Atomic Orbitals (LCAO) method, considering the molecular wave function as a linear combination of all valence orbitals, i.e., 2s, 2p, 2p, 2p orbitals for C, N, and O atoms and 1s orbital for H atoms. Regarding the diagonal matrix elements (also known as on-site energies), we introduce a novel parameterization. For the non-diagonal matrix elements referring to neighboring atoms, we employ the Slater–Koster two-center interaction transfer integrals. We use Harrison-type expressions with factors slightly modified relative to the original. We compare our LCAO predictions for the ionization and excitation energies of heterocycles with those obtained from Ionization Potential Equation of Motion Coupled Cluster with Singles and Doubles (IP-EOMCCSD)/aug-cc-pVDZ level of theory and Completely Normalized Equation of Motion Coupled Cluster with Singles, Doubles, and non-iterative Triples (CR-EOMCCSD(T))/aug-cc-pVDZ level of theory, respectively, (vertical values), as well as with available experimental data. Similarly, we calculate the transfer integrals between subsequent base pairs, to be used for a Tight-Binding (TB) wire model description of charge transfer and transport along ideal or deformed B-DNA. Taking into account all valence orbitals, we are in the position to treat deflection from the planar geometry, e.g., DNA structural variability, a task impossible for the plane Hückel approach (i.e., using only 2p orbitals). We show the effects of structural deformations utilizing a 20mer evolved by Molecular Dynamics.

Structural properties of MeF2 (Me=Ca, Sr, Ba) crystals doped with rare-earth ions RE3+/ RE2+: ab initio route

In this paper, we simulated the structural properties of the cubic impurity centers RE3+/RE2+ (RE = Rare earth) in fluorites crystals MeF2 (Me = Ca, Sr, Ba) by ab initio route. Doping fluorite crystals with the rare-earth ion RE3+/RE2+, this replaces the cation Me2+ and determines distortions of the MeF2 crystal lattice near the impurity center. These distortions characterized by radial and angular displacements of anions were investigated by ab initio method. We have employed the MO LCAO approach, in the frame of DFT method as it is implemented in the CRYSTAL17 software for periodic systems, using the hybrid functional PBE0. Since the main contribution to the crystal field on the rare-earth impurity ions comes from its nearest neighbors, the distance “rare-earth ion – ligand” and the radial and angular coordinates of the RE3+/RE2+ ions at several coordination spheres of anions near the impurity centers are calculated. Also, the changes in the local compressibility of the cationic and anionic sublattice next to the impurity ion are determined. The obtained results were discussed, compared with available experimental data and other theoretical results, and the agreement is quite satisfactory.

Topology of the Electron Density and of Its Laplacian from Periodic LCAO Calculations on f-Electron Materials: The Case of Cesium Uranyl Chloride.

The chemistry of f-electrons in lanthanide and actinide materials is yet to be fully rationalized. Quantum-mechanical simulations can provide useful complementary insight to that obtained from experiments. The quantum theory of atoms in molecules and crystals (QTAIMAC), through thorough topological analysis of the electron density (often complemented by that of its Laplacian) constitutes a general and robust theoretical framework to analyze chemical bonding features from a computed wave function. Here, we present the extension of the Topond module (previously limited to work in terms of s-, p- and d-type basis functions only) of the Crystal program to f- and g-type basis functions within the linear combination of atomic orbitals (LCAO) approach. This allows for an effective QTAIMAC analysis of chemical bonding of lanthanide and actinide materials. The new implemented algorithms are applied to the analysis of the spatial distribution of the electron density and its Laplacian of the cesium uranyl chloride, CsUOCl, crystal. Discrepancies between the present theoretical description of chemical bonding and that obtained from a previously reconstructed electron density by experimental X-ray diffraction are illustrated and discussed.

Optical features of ligated semiconducting quantum dots subjected to an electric field

AbstractSemiconducting quantum dots (SQDs) are attractive materials for optoelectronic applications due to the size‐tunable band gaps, colloidal stability, and low‐cost solution processability. In this article, we investigate the modulations in the optical features of COOH + NH3 capped tetrahedral‐shape CdSe SQD in response to the external electric field. We base our calculations on an approach which combines the linear combination of atomic orbitals (LCAO) real‐time‐propagation‐time‐dependent density functional theory (rt‐TDDFT) technique (LCAO‐rt‐TDDFT) and transition contribution maps (two‐dimensional visualization of Kohn–Sham single electron–single hole transitions). Among other insights, our study indicates that electric field causes a redistribution of the oscillator strengths, early onset of absorption, and ligand‐derived trap states near the top of the valence band. These theoretical findings explain the origins of the electric field induced optical features of ligated SQDs.

The correlation between experimental polarized Raman spectra and their density functional theory prediction in the LCAO framework: The R3c LiNbO3 crystal as a test case

AbstractIn this study, the polarized Raman spectra of the R3c LiNbO3 crystal are used as a benchmark test for density functional theory (DFT) full periodic boundary conditions linear combination of atomic orbitals (LCAO) calculation of the Raman tensors, according to the implementation in the CRYSTAL software. The theoretical approach used proved to provide excellent results regarding wavenumbers and relative intensities predictions for the transverse optical modes of both A1 and E symmetry, considerably improving over previously reported data based on the plane waves approach. Overall, the present investigation demonstrates that the LCAO approach, as implemented in the CRYSTAL software, gives excellent results regarding the calculation of Raman tensors and polarized Raman spectra. The possibility to put in correspondence the individual Raman tensors components and bands intensities in the different back‐scattering experimental configurations revealed that the computed Raman tensors are very accurate, not only considering their average values (tensors invariant in the combination suitable for the description of Raman scattering of isotropic materials) but also when the tensors individual components are considered. Based on the present results, a reassignment of the E (TO) modes of the R3c LiNbO3 crystal is proposed, in particular for the 2 E and 9 E modes, which have been a matter of discussion in the recent years.

Plasmonic properties of nanohybrids made of metallic nanoring and benzene molecules

AbstractPlasmon‐molecule coupling has recently been attracted great research interest in material science and quantum optics with a multitude of potential applications. In this work, using the linear combination of atomic orbitals real‐time‐propagation rt‐time dependent density functional theory (TDDFT) technique (LCAO‐rt‐TDDFT) and transition contribution maps, we investigate the optical and plasmonic features of a hybrid nanosystem comprising an elliptical‐shaped metal nanoring (ENR) and a couple of benzene molecules (located in close proximity at both sides of the ENR short axis and/or long axis). We find that pristine ENR exhibits localized surface plasmon resonances (LSPRs) and plasmon‐molecule coupling in hybrid nanosystems is highly sensitive to the location of the molecules. More specifically, when benzene molecules are placed at both sides of the ENR short axis, the dominant peak in the photoabsorption spectrum sustains its LSPR features with a significant enhancement in intensity. On the other hand, in an alternative situation where benzene molecules are placed at both sides of the ENR long axis, the dominant peak in the photoabsorption spectrum splits into three with diminishing LSPR features.

A systematic construction of Gaussian basis sets for the description of laser field ionization and high-harmonic generation.

A precise understanding of mechanisms governing the dynamics of electrons in atoms and molecules subjected to intense laser fields has a key importance for the description of attosecond processes such as the high-harmonic generation and ionization. From the theoretical point of view, this is still a challenging task, as new approaches to solve the time-dependent Schrödinger equation with both good accuracy and efficiency are still emerging. Until recently, the purely numerical methods of real-time propagation of the wavefunction using finite grids have been frequently and successfully used to capture the electron dynamics in small one- or two-electron systems. However, as the main focus of attoscience shifts toward many-electron systems, such techniques are no longer effective and need to be replaced by more approximate but computationally efficient ones. In this paper, we explore the increasingly popular method of expanding the wavefunction of the examined system into a linear combination of atomic orbitals and present a novel systematic scheme for constructing an optimal Gaussian basis set suitable for the description of excited and continuum atomic or molecular states. We analyze the performance of the proposed basis sets by carrying out a series of time-dependent configuration interaction calculations for the hydrogen atom in fields of intensity varying from 5 × 1013 W/cm2 to 5 × 1014 W/cm2. We also compare the results with the data obtained using Gaussian basis sets proposed previously by other authors.

Efficient electronic structure calculations for extended systems of coupled quantum dots using a linear combination of quantum dot orbitals method

We present a novel "linear combination of atomic orbitals"-type of approximation, enabling accurate electronic structure calculations for systems of up to 20 or more electronically coupled quantum dots. Using realistic single quantum dot wavefunctions as basis to expand the eigenstates of the heterostructure, our method shows excellent agreement with full 8-band $\boldsymbol{k}\cdot\boldsymbol{p}$ calculations, exemplarily chosen for our benchmarking comparison, with an orders of magnitude reduction in computational time. We show that, in order to correctly predict the electronic properties of such stacks of coupled quantum dots, it is necessary to consider the strain distribution in the whole heterostructure. Edge effects determine the electronic structure for stacks of $\lesssim$ 10 QDs, after which a homogeneous confinement region develops in the center. The overarching goal of our investigations is to design a stack of vertically coupled quantum dots with an intra-band staircase potential suitable as active material for a quantum-dot-based quantum cascade laser. Following a parameter study in the In$_{x}$Ga$_{1-x}$As/GaAs material system, varying quantum dot size, material composition and inter-dot coupling strength, we show that an intra-band staircase potential of identical transitions can in principle be realized. A species library we generated for over 800 unique quantum dots provides easy access to the basis functions required for different realizations of heterostructures. In the associated manuscript entitled "Room Temperature Lasing of Terahertz Quantum Cascade Lasers Based on a Quantum Dot Superlattice", we investigate room temperature lasing of a terahertz quantum cascade laser based on a two-quantum-dot unit cell superlattice.

Quantum Molecular Polyhedra and Atomic Populations

The present paper uses the LCAO MO theory formalism. The structure of the first order electronic density function is decomposed in two kinds of quantum polyhedra to discuss the behavior of quantum atomic populations. Among the many aspects one can consider about atomic populations here, the quantum mechanical structure of the density function is taken as the most important characteristic to think about. Apart of the usual one-electron basis set, centered in the molecular atoms, there is also discussed the possibility that the three-dimensional space where the molecular structures are described can be also the site of basis functions centered in points non-coincident with atomic positions.

The electronic structure studies of hybrid h-BNC sheets based on a semi-empirical Hamiltonian

The quest for modulating the wide bandgap of a pristine h-BN sheet for device-related applications has prompted the present study of ternary 2-dimensional sheets, h-BNC, that contain carbon domains of different shapes and sizes embedded in the h-BN network. The structural stability and electronic properties of hybrid h-BNC sheets containing rectangular-, circular-, hexagonal-, and triangular-shaped carbon domains are investigated using a real-space electronic structure method, where an environment-dependent semi-empirical Hamiltonian is used within a framework of a linear combination of atomic orbitals and self-consistent charge calculations. This method allows a study of larger carbon domains embedded in the h-BN matrix beyond what is possible via first-principles calculations, and thus serves to complement previous theoretical studies on hybrid h-BNC systems. The electronic density of states reveals mid-gap states for all h-BNC sheets, suggesting a narrowing of the energy gap compared to the pristine h-BN sheet. They arise from the breaking of the hexagonal symmetry due to bond distortions at the interface between the carbon domain and the h-BN network. The features of such mid-gap states strongly depend on the size and shape of carbon domains and the type of bonding (C-N, C-B, or a mixture of both) at the interface. The hybrid h-BNC sheet containing rectangular carbon domains switches from a semiconductor to a gapless semi-metal-like, and to a metal-like system as the size of the carbon domain increases. The energy gap of the h-BNC sheet containing hexagonal carbon domains exhibits a power-law decrease with the size of the carbon domains. On the other hand, the energy gap of the hybrid h-BN sheet containing circular carbon domains oscillates as the size of the carbon domain is increased. For electron-rich hybrid h-BNC sheets containing triangular graphene domains and C-N interfaces, a p-type-like behaviour is obtained. On the other hand, for electron-deficient hybrid h-BNC sheets, containing triangular graphene domains and C-B interfaces, the electronic behaviour switches from a p-type-like semiconductor to a metal-like behaviour as the number of carbon atoms increase. The top of the valence band was found to be half-filled for all triangular domains. The results obtained here may pave the way for novel device concepts based on h-BNC sheets.

Mutual Neutralization in Li++H−/D− and Na++H−/D− Collisions: Implications of Experimental Results for Non-LTE Modeling of Stellar Spectra

Abstract Advances in merged-beams instruments have allowed experimental studies of the mutual neutralization (MN) processes in collisions of both Li+ and Na+ ions with D− at energies below 1 eV. These experimental results place constraints on theoretical predictions of MN processes of Li+ and Na+ with H−, important for non-LTE modeling of Li and Na spectra in late-type stars. We compare experimental results with calculations for methods typically used to calculate MN processes, namely the full quantum (FQ) approach, and asymptotic model approaches based on the linear combination of atomic orbitals (LCAO) and semiempirical (SE) methods for deriving couplings. It is found that FQ calculations compare best overall with the experiments, followed by the LCAO, and the SE approaches. The experimental results together with the theoretical calculations, allow us to investigate the effects on modeled spectra and derived abundances and their uncertainties arising from uncertainties in the MN rates. Numerical experiments in a large grid of 1D model atmospheres, and a smaller set of 3D models, indicate that neglect of MN can lead to abundance errors of up to 0.1 dex (26%) for Li at low metallicity, and 0.2 dex (58%) for Na at high metallicity, while the uncertainties in the relevant MN rates as constrained by experiments correspond to uncertainties in abundances of much less than 0.01 dex (2%). This agreement for simple atoms gives confidence in the FQ, LCAO, and SE model approaches to be able to predict MN with the accuracy required for non-LTE modeling in stellar atmospheres.

Electronic structure of tantalum dichalcogenide using Compton scattering technique and density functional theory

Compton spectroscopy measurements on tantalum dichalcogenide TaS0.5Se1.5 using a 100 mCi Am-241 γ-ray Compton spectrometer are presented. Experimental Compton profile (CP) is compared with those computed using linear combination of atomic orbitals (LCAO) method within the density functional theory (DFT) and hybrid functionals of Hartree-Fock and DFT. Hybrid functional namely WC1LYP based CP is found in better accordance with the experimental CP. The relative nature of bonding in TaS0.5Se1.5 and TaSSe is interpreted by scaling the experimental CP on equal-valence-electron-density and Mulliken's population analysis. Origin of metallic behaviour of TaS0.5Se1.5 is discussed in terms of energy bands and density of states. It is seen that the 5d states of Ta dominate around the Fermi level leading to metallic character of the compound. Fermi surface topology and optical response like dielectric constants, absorption coefficients, etc. of TaS0.5Se1.5 are also described using modified Becke-Johnson theory of full-potential linearized augmented plane wave approach.

Effects of the Hubbard U correction on the electronic and magnetic properties of the tetragonal V2P2 sheet.

A recent theoretical work predicted the orthorhombic phase of the V2P2 sheet with the half-metallic electronic property using a linear combination of atomic orbitals (LCAO) basis set based on density functional theory (DFT). However, in the plane-wave DFT method, the tetragonal (t) V2P2 phase is the ground state structure. The total energy of the optimized tetragonal V2P2 is 0.91 eV per cell lower than that of the orthorhombic phase. Herein, we investigated the effects of Hubbard U correction onthe electronic, magnetic, and adsorption properties of the t-V2P2 sheet. The t-V2P2 sheet is found to be dynamically and mechanically stable. The t-V2P2 sheet prefers an antiferromagnetic ground state with an indirect narrowed bandgap of 0.23 eV. The estimated electron mobility in the t-V2P2 sheet at room temperature is approximately 24 times that of a hole. The t-V2P2 sheet exhibits a sizable magnetic anisotropy (MAE) of 69.63 μeV per V atom with in-plane magnetization. Mean-field approximation based on the 2D classical Heisenberg model predicts a high Néel temperature (TN) of the t-V2P2 sheet up to 1263 K. The Li atom adsorption on the t-V2P2 sheet shows a transition from semiconductor to metal. Also the Li–V2P2 system has a residual integer magnetic moment of 1 μB. Due to strong steric coulomb repulsion, the minimum diffusion energy barrier (Ea) for the Li-ion on the t-V2P2 surface is high enough to make the Li atom immobile. Our findings demonstrate the potential of the t-V2P2 sheet for antiferromagnetic spintronics and sensing applications.

The Sternheimer approach to all-electron real-space density-functional perturbation theory with atomic basis set

We present an efficient perturbative method to get the response density matrix using localized non-orthogonal basis sets. This scheme is based on the solution of the coupled perturbed self-consistent field equation with the Sternheimer approach, which only requires the occupied states and avoids a sum over unoccupied states. We present a complete derivation of the Sternheimer approach to perturbation theory within the framework of the linear combination of atomic orbitals. To demonstrate the capabilities of this method, we have implemented it in the all-electron Fritz Haber Institute ab initio molecular simulation package and applied it to benchmark molecules. For the response properties with respect to the atomic displacement and to the homogeneous electric field, the results are in excellent agreement with those of the previous traditional method and fully validate this Sternheimer approach.

Investigating the naturally occurring forms of TiO2 on electronic and optical properties using OLCAO-MGGA-TBO9: a hybrid DFT study

In this paper, a detailed study and analysis on the electronic and optical properties of anatase, rutile and brookite titanium dioxide (TiO2) which are the naturally occurring phases of TiO2 have been carried out. We have obtained these properties using the self-consistent orthogonalized linear combination of atomic orbitals with meta-generalized gradient approximation (MGGA) and Tran and Blaha (TBO9) as exchange–correlation under the framework of density functional theory. Obtained results on band gap value (E g), dielectric constant and refractive index as calculated by considering the optimal value of c (system-dependent parameter) have been analyzed statistically and are found to be much closer to the experimental values and are better than the other approaches published in the literature. It is seen that optical absorption for all the three phases of TiO2 occurs in UV region of EM spectrum. Using statistical analysis in correlation with other effective methods such as mBJ, GGA + U, GGA + Ud + Up, LSD + U, GW and HSE06 functional, it is found that MGGA-TB09 gives a better description of electronic structure and optical properties with less computation time. This work provides good understanding of electronic and optical properties of TiO2, stems a foundation for its possible applications in photo catalytic activities of dye sensitized solar cells.

Mathematical modeling of the electronic structure of Titanium dioxide \((TiO_2 )_6\) nanoparticles

The calculation of the number of atoms of the given dimensional nanoparticle, composed of different type atoms has been researched in this work. The calculations have been carried out for nanoparticles of titanium dioxide. Theoretical visual models have been configured, and quantum – mechanical calculations have been carried out for \((TiO_2 )_6\) nanoparticle. The calculations for titanium dioxide nanoparticle have been carried out on the basis of Gaussian atomic orbitals. Besides, Gaussian functions have been used as atomic orbitals. The numerical values of unknown coefficients of the linear combination of atomic orbitals of the atoms of the titanium nanoparticle have been found from the solution of Hartree–Fock–Roothaan (HFR) equations.The values of orbital energies, ionization potential, and the total electronic energy of titanium dioxide nanoparticles have been determined. The calculations show that ,titanium dioxide nanoparticle is tough, electrophile, and stable dielectric, material. The effective charge of atoms have been calculated, and the theoretical visual mode of titanium dioxide nanopartical have been constructed.

First Principle Investigation of Electronic, Transport, and Bulk Properties of Zinc-Blende Magnesium Sulfide

We have studied electronic, structural, and transport properties of zinc-blende magnesium sulfide (zb-MgS). We employed a local density approximation (LDA) potential and the linear combination of atomic orbitals (LCAO) method. Our computational method is able to reach the ground state of a material, as dictated by the second theorem of density functional theory (DFT). Consequently, our findings have the physical content of DFT and agree with available, corresponding experimental ones. The calculated band gap of zb-MgS, a direct gap equal to 4.43 eV, obtained at the experimental lattice constant of 5.620 Å, completely agrees with the experimental band gap of 4.45 ± 0.2 eV. We also report total (DOS) and partial (pDOS) densities of states, electron and hole effective masses, the equilibrium lattice constant, and the bulk modulus. The calculated pDOS also agree with the experiment for the description of the states at the top and the bottom of the valence and conduction bands, respectively.

The Effect of Pressure on the Structure and Electronic Properties of Hydrated Calcium Carbonates

This paper studies the effect of pressure on the structure and electronic properties of CaCO3-H2O and CaCO3-6H2O crystalline hydrates. The study is based on the density functional theory (DFT) and the linear combination of atomic orbitals (LCAO) method. Calculations are performed using the CRYSTAL17 software package and the PBE gradient functional. The calculated lattice parameters of hydrated calcium carbonates and their dependence on external hydrostatic pressure are shown to be in good agreement with the available experimental measurements. Dependencies of linear compressibility on the direction are obtained using the calculated pressure dependencies of the structural parameters. It is demonstrated that the linear compressibility of calcium carbonate hexahydrate, in contrast to calcium carbonate monohydrate, is highly anisotropic (the smallest and largest compressibility values are correlated as K max /K min ~ 4). In this case, the maximum compressibility is located between crystallographic axes (between the axes a and c) and not along the crystallographic axes. The bulk modulus for the monohydrate (CaCO3-6H2O) is greater than for the hexahydrate (CaCO3-6H2O). Total and partial densities of electronic states for CaCO3-H2O and CaCO3-6H2O are calculated. Also, dependencies of the band gap width on pressure for hydrated calcium carbonates are established. It is shown that, with increasing pressure, the increase of the band gap is greater for CaCO3-6H2O than for CaCO3-H2O.

Topological analysis of chemical bonding in the layered FePSe3 upon pressure-induced phase transitions.

Two pressure-induced phase transitions have been theoretically studied in the layered iron phosphorus triselenide (FePSe3 ). Topological analysis of chemical bonding in FePSe3 has been performed based on the results of first-principles calculations within the periodic linear combination of atomic orbitals (LCAO) method with hybrid Hartree-Fock-DFT B3LYP functional. The first transition at about 6 GPa is accompanied by the symmetry change from to C2/m, whereas the semiconductor-to-metal transition (SMT) occurs at about 13 GPa leading to the symmetry change from C2/m to . We found that the collapse of the band gap at about 13 GPa occurs due to changes in the electronic structure of FePSe3 induced by relative displacements of phosphorus or selenium atoms along the c-axis direction under pressure. The results of the topological analysis of the electron density and its Laplacian demonstrate that the pressure changes not only the interatomic distances but also the bond nature between the intralayer and interlayer phosphorus atoms. The interlayer P-P interactions are absent in two non-metallic FePSe3 phases while after SMT the intralayer P-P interactions weaken and the interlayer P-P interactions appear.

Unfolding optical transition weights of impurity materials for first-principles LCAO electronic structure calculations

A method to analyze optical transitions is developed by combining the Kubo-Greenwood formula with the unfolding method to construct an unfolded electronic band structure with optical transition weights, which allows us to investigate how optical transitions are perturbed by imperfections such as impurity, vacancy, and structural distortions. Based on the Kubo-Greenwood formula, we first calculate frequency-dependent optical conductivity based on the first-principles electronic structure calculations using the linear combinations of atomic orbitals. Benefiting from the atomic orbital basis sets, the frequency-dependent optical conductivity can be traced back to their individual components before summations over all of $k$ points and bands. As a result, optical transition weights of the material can be put on the unfolded electronic band structure to show contributions at different $k$ points and bands. This method is especially useful to study the effects of broken symmetry in the optical transitions due to presence of impurities in the materials. As a demonstration, decomposed optical transition weights of a monolayer Si-doped graphene are shown in the electronic band structure.