Local Orbital Basis Research Articles

NMR shieldings from density functional perturbation theory: GIPAW versus all-electron calculations

We present a benchmark of the density functional linear response calculation of NMR shieldings within the gauge-including projector-augmented-wave method against all-electron augmented-plane-wave+local-orbital and uncontracted Gaussian basis set results for NMR shieldings in molecular and solid state systems. In general, excellent agreement between the aforementioned methods is obtained. Scalar relativistic effects are shown to be quite large for nuclei in molecules in the deshielded limit. The small component makes up a substantial part of the relativistic corrections.

First principles electronic structure and magnetic properties of inverse Heusler alloys X2YZ(X=Cr; Y=Co, Ni; Z=Al, Ga, In, Si, Ge, Sn, Sb)

Using augmented plane wave+local orbital basis, we have calculated the electronic structure and magnetic properties of X2YZ (X=Cr; Y=Co and Ni; Z=Al, Ga, In, Si, Ge, Sn, Sb) inverse Heusler alloys from first principles. We employ the Hg2CuTi type L21 structure which indeed provides the low energy solution. The calculated total magnetic moments considered here follow the generalized Slater-Pauling behavior (Zt−24) except Cr2NiSb, which shows (Zt−28) instead. These materials show a systematic trend of ferrimagnetic Cr-Cr exchange interaction which increases with the number of valence electrons. The half-metallic and ferrimagnetic behaviors make them promising candidates for spintronic materials and devices.

Experimental and Theoretical High-Energy-Resolution X-ray Absorption Spectroscopy: Implications for the Investigation of the Entatic State.

High-energy-resolution-fluorescence-detected X-ray absorption near-edge structure (HERFD-XANES) spectroscopy is shown to be a sensitive tool to investigate the electronic changes of copper complexes induced by geometric distortions caused by the ligand backbone as a model for the entatic state. To fully exploit the information contained in the spectra gained by the high-energy-resolution technique, (time-dependent) density functional theory calculations based on plane-wave and localized orbital basis sets are performed, which in combination allow the complete spectral range from the prepeak to the first resonances above the edge step to be covered. Thus, spectral changes upon oxidation and geometry distortion in the copper N-(1,3-dimethylimidazolidin-2-ylidene)quinolin-8-amine (DMEGqu) complexes [CuI(DMEGqu)2](PF6) and [CuII(DMEGqu)2](OTf)2·MeCN can be accessed.

Mulliken Occupation as the Indicator of Transition to Superconducting State in SrFe2As2and BaFe2As2

Basing on the ab initio calculations performed within full potential local orbital minimum basis method, the Mulliken occupation of the Sr 5s (n5s) and Ba 6p (n6p) states can serve as an indicator of transition to the superconducting state in doped SrFe2As2 and BaFe2As2 compounds where the iron was substituted with cobalt or in the pristine compounds under pressure. The estimated pressure, at which both compounds exhibit superconductivity based on Sr 5s and Ba 6p occupation is in good agreement with the recently published experimental data.

Electronic and optical properties of hexathiapentacene in the gas and crystal phases

Using density functional theory (DFT) and its time-dependent (TD) extension, the electronic and optical properties of the hexathiapentacene (HTP) molecule, a derivative of pentacene (PNT) obtained by symmetric substitution of the six central H atoms with S atoms, are investigated for its gas and solid phases. For the molecular structure, all-electron calculations are performed using a Gaussian localized orbital basis set in conjunction with the Becke three-parameter Lee-Yang-Parr (B3LYP) hybrid exchange-correlation functional. Electron affinities, ionization energies, quasiparticle energy gaps, optical absorption spectra, and exciton binding energies are calculated and compared with the corresponding results for PNT, as well as with the available experimental data. The DFT and TDDFT results are also validated by performing many-body perturbation theory calculations within the $GW$ and Bethe-Salpeter equation formalisms. The functionalization with S atoms induces an increase of both ionization energies and electron affinities, a sizable reduction of the fundamental electronic gap, and a redshift of the optical absorption onset. Notably, the intensity of the first absorption peak of HTP falling in the visible region is found to be nearly tripled with respect to the pure PNT molecule. For the crystal structures, pseudopotential calculations are adopted using a plane-wave basis set together with the Perdew-Burke-Ernzerhof exchange-correlation functional empirically corrected in order to take dispersive interactions into account. The electronic excitations are also obtained within a perturbative B3LYP scheme. A comparative analysis is carried out between the ground-state and excited-state properties of crystalline HTP and PNT linking to the findings obtained for the isolated molecules.

Magnetic properties and stability of Cu3V2O8 compound in the different phases

The magnetic and thermodynamic properties of Cu3V2O8 compound in four structures (P−1, P21/c, P21/m and Cmca) are reported. The calculations are performed by using the Full-Potential Local Orbital Minimum Basis (FPLO) and Vienna ab initio Simulation Package (VASP) methods. We have applied the local density approximation (LDA) with the generalized gradient corrections (GGA). The effect of electron correlations was also included in GGA+U approximation. The thermodynamic properties were obtained in the quasi-harmonic Debye–Grüneisen model using the equation of states (EOS) in the form of Poirier–Tarantola. Our ab-intio results indicate that α (P−1) phase is stable below 1.87GPa, β (P21/c) exists in the region 1.87<P<7.42GPa and γ (Cmca) is observed above P=7.42GPa. Our results have shown that in GGA and GGA+U the antyferromagnetic configuration is stable in P−1, P21/c and P21/m structures. The calculated values of gaps in P−1, P21/c crystal structures are close to the experimental data.

Electronic structure and thermodynamic properties of platinum–lead oxides PbPt2O4 and Pb2PtO4 by ab initio methods

ABSTRACTWe report the electronic structure and thermodynamic properties of platinum–lead oxides PbPt2O4 and Pb2PtO4. The band structure is calculated by the full-potential, full-relativistic local orbital minimum basis (FPLO) method in the local density approximation, including the generalized gradient approximation (GGA) and GGA+U, respectively. The values of gaps in Pb2PtO4 were estimated as 0.47, 1.02, and 0.52 eV in GGA, GGA+U, and the full-relativistic GGA, respectively. Pb2PtO4 is a semiconductor, and PbPt2O4 is a metallic conductor with the small values of the density of states at the Fermi level. The effect of chemical disorder in Pb2−xPt1+xO4 alloys is studied by the relaxation of the volume, shape, and the positions of atoms in the unit cell, using the Vienna Ab initio Simulation Package (VASP) method. The thermodynamic properties are computed in the quasi-harmonic Debye–Grüneisen model. The dependence of the thermodynamic parameters on the pressure and temperature is also presented.

Ab-initio study of electronic structure and thermodynamic properties of aurates BaAu2O4 and SrAu2O4

Abstract Using ab-initio the Full-Potential Local Orbital minimum basis (FPLO) and Vienna Simulation Package (VASP) methods in the framework of the local density functional theory within the generalized gradient approximation (GGA) the band structure and thermodynamic properties of aurates BaAu 2 O 4 , SrAu 2 O 4 and BaSrAu 2 O 4 have been studied. Ab-inito calculations have shown that these systems are semiconductors with the gap between 1.42 and 1.65 eV. The values of gaps decrease with the increase of the volume. The thermodynamic parameters (bulk modulus and Debye temperature) are calculated in the quasi-harmonic Debye–Gruneisen model. Both ab-initio methods give the similar results.

Electronic structure, thermodynamic properties and hydrogenation of LaPtIn and CePtIn compounds by ab-initio methods

The electronic structures and thermodynamic properties of LaPtIn and CePtIn are studied by means of ab-initio full-relativistic full-potential local orbital basis (FPLO) method within densities functional (DFT) methodologies. We have also examined the influence of hydrogen on the electronic structure and stability of CePtInH and LaPtInH systems. The positions of the hydrogen atoms have been found from the minimum of the total energy.Our calculations have shown that band structure and topology of the Fermi surfaces changed significantly during the hydrogenation. The thermodynamic properties (bulk modulus, Debye temperatures, constant pressure heat capacity) calculated in quasi-harmonic Debye-Grüneisen model are in a good agreement with the experimental data. We have applied different methods of the calculation of the equation of states (EOS) (Murnaghan, Birch-Murnaghan, Poirier–Tarantola, Vinet). The thermodynamic properties are presented for the pressure 0<P<9GPa and the temperature range 0<T<300K.

Nonequilibrium spin texture within a thin layer below the surface of current-carrying topological insulatorBi2Se3: A first-principles quantum transport study

We predict that unpolarized charge current injected into a ballistic thin film of prototypical topological insulator (TI) Bi$_2$Se$_3$ will generate a {\it noncollinear spin texture} $\mathbf{S}(\mathbf{r})$ on its surface. Furthermore, the nonequilibrium spin texture will extend into $\simeq 2$ nm thick layer below the TI surfaces due to penetration of evanescent wavefunctions from the metallic surfaces into the bulk of TI. Averaging $\mathbf{S}(\mathbf{r})$ over few \AA{} along the longitudinal direction defined by the current flow reveals large component pointing in the transverse direction. In addition, we find an order of magnitude smaller out-of-plane component when the direction of injected current with respect to Bi and Se atoms probes the largest hexagonal warping of the Dirac-cone dispersion on TI surface. Our analysis is based on an extension of the nonequilibrium Green functions combined with density functional theory (NEGF+DFT) to situations involving noncollinear spins and spin-orbit coupling. We also demonstrate how DFT calculations with properly optimized local orbital basis set can precisely match putatively more accurate calculations with plane-wave basis set for the supercell of Bi$_2$Se$_3$.

First-principles study of magnetic, electronic, elastic and thermal properties of GdFe2

The magnetic, electronic, elastic and thermal properties of GdFe2 have been investigated under ambient and hydrostatic pressure, in the pressure range 0–100 GPa using the Full Potential Nonorthogonal Local-Orbital minimum basis method (FPLO) within the generalized gradient approximation (GGA) and the local spin density approximation (LSDA). We have calculated the lattice constant, bulk modulus and its first pressure derivative. The bulk modulus using GGA is 104.05 GPa in fair agreement with experimental results. The total magnetic moment of GdFe2 using GGA and LSDA are 3.65 and 4.38 μB at ambient pressure respectively. The pressure effect on the partial, total magnetic moment and total DOS is investigated under high pressure the results indicate that GdFe2 is metallic under all applied pressure. The magnetic and thermal heat capacity and entropy for GdFe2 have been studied using results from ab-initio calculations and proper models. The magnetic heat capacity contributes only 0.5 J/g K at 793 K and the thermal heat capacity is saturated at 140 J/mol K above Debye temperature. The maximum contribution of the magnetic entropy to the total entropy is 0.8 J/g K in the studied temperature range.

Linear-Scaling Coupled Cluster with Perturbative Triple Excitations: The Divide-Expand-Consolidate CCSD(T) Model.

We propose a reformulation of the traditional (T) triples correction to the coupled cluster singles and doubles (CCSD) energy in terms of local Hartree-Fock (HF) orbitals such that its structural form aligns with our recently developed linear-scaling divide-expand-consolidate (DEC) coupled cluster family of local correlation methods. In a DEC-CCSD(T) calculation, a basis of local occupied and virtual HF orbitals is used to partition the correlated calculation on the full system into a number of independent atomic fragment and pair fragment calculations, each performed within a truncated set of the complete orbital space. In return, this leads to a massively parallel algorithm for the evaluation of the DEC-CCSD(T) correlation energy, which formally scales linearly with the size of the full system and has a tunable precision with respect to a conventional CCSD(T) calculation via a single energy-based input threshold. The theoretical developments are supported by proof of concept DEC-CCSD(T) calculations on a series of medium-sized molecular systems.

A first-principles study of PuN (0 0 1) surface properties

The properties of PuN (001) surface have been investigated using all-electron full-potential linearized augmented plane wave plus local orbitals basis (FP-LAPW+lo) method at both scalar- and fully- relativistic levels, with and without spin-polarization. The ground state of PuN (001) surface is found to be ferromagnetic with spin–orbit coupling (FM+SO). At the ground state, the semi-infinite surface energy and average work function for PuN (001) surface are predicted to be 1.12J/m2 and 2.62eV, respectively. At three layers of formula units and beyond, the PuN (001) surface properties including total energies per N-formula unit, surface energy, work function and magnetic moment converge, indicating that a 3-layer slab can be used to model PuN (001) surface to a good approximation. The localization of the 5f electron states is pronounced at the top surface layer while bulk-like behavior is exhibited at the second and deeper layers.

Electronic structure and thermodynamic properties of Cu3V2O8 compound

The electronic structure and thermodynamic properties of Cu3V2O8 compound for three structures (P-1, P21/c and Cmca) are reported. The calculations are performed by using full-potential local orbital minimum basis method. The total electronic densities of states for all structures have the similar shape but the details are different. The thermodynamic properties (the bulk modulus B, Gibbs free energy, Debye temperature ΘD) are calculated in quasiharmonic Debye–Grüneisen model using the equation of states in the form of Murnaghan, Birch–Murnaghan, Poirier–Tarantola and Vinet. Our ab initio results indicate that α(P-1) phase is stable below 839 K, β(P21/c) and γ(Cmca) phases can exist in the region of 839 K < T < 875 K, however above T = 875 K only γ(Cmca) phase is observed.

In silico infrared and Raman spectroscopy under pressure: the case of CaSnO3 perovskite.

The CaSnO3 perovskite is investigated under geochemical pressure, up to 25 GPa, by means of periodic ab initio calculations performed at B3LYP level with local Gaussian-type orbital basis sets. Structural, elastic, and spectroscopic (phonon wave-numbers, infrared and Raman intensities) properties are fully characterized and discussed. The evolution of the Raman spectrum of CaSnO3 under pressure is reported to remarkably agree with a recent experimental determination [J. Kung, Y. J. Lin, and C. M. Lin, J. Chem. Phys. 135, 224507 (2011)] as regards both wave-number shifts and intensity changes. All phonon modes are symmetry-labeled and bands assigned. The single-crystal total spectrum is symmetry-decomposed into the six directional spectra related to the components of the polarizability tensor. The infrared spectrum at increasing pressure is reported for the first time and its main features discussed. All calculations are performed using the Crystal14 program, taking advantage of the new implementation of analytical infrared and Raman intensities for crystalline materials.

First-principles investigation on Rydberg and resonance excitations: A case study of the firefly luciferin anion.

The optical properties of an isolated firefly luciferin anion are investigated by using first-principles calculations, employing the many-body perturbation theory to take into account the excitonic effect. The calculated photoabsorption spectra are compared with the results obtained using the time-dependent density functional theory (TDDFT) employing the localized atomic orbital (AO) basis sets and a recent experiment in vacuum. The present method well reproduces the line shape at the photon energy corresponding to the Rydberg and resonance excitations but overestimates the peak positions by about 0.5 eV. However, the TDDFT-calculated positions of some peaks are closer to those of the experiment. We also investigate the basis set dependency in describing the free electron states above vacuum level and the excitons involving the transitions to the free electron states and conclude that AO-only basis sets are inaccurate for free electron states and the use of a plane wave basis set is required.

Ab Initio Simulation of Charge Transfer at the Semiconductor Quantum Dot/TiO2 Interface in Quantum Dot‐Sensitized Solar Cells

Quantum dot‐sensitized solar cells (QDSSCs) have emerged as a promising solar architecture for next‐generation solar cells. The QDSSCs exhibit a remarkably fast electron transfer from the quantum dot (QD) donor to the TiO2 acceptor with size quantization properties of QDs that allows for the modulation of band energies to control photoresponse and photoconversion efficiency of solar cells. To understand the mechanisms that underpin this rapid charge transfer, the electronic properties of CdSe and PbSe QDs with different sizes on the TiO2 substrate are simulated using a rigorous ab initio density functional method. This method capitalizes on localized orbital basis set, which is computationally less intensive. Quite intriguingly, a remarkable set of electron bridging states between QDs and TiO2 occurring via the strong bonding between the conduction bands of QDs and TiO2 is revealed. Such bridging states account for the fast adiabatic charge transfer from the QD donor to the TiO2 acceptor, and may be a general feature for strongly coupled donor/acceptor systems. All the QDs/TiO2 systems exhibit type II band alignments, with conduction band offsets that increase with the decrease in QD size. This facilitates the charge transfer from QDs donors to TiO2 acceptors and explains the dependence of the increased charge transfer rate with the decreased QD size.

Density functional theory based generalized effective fragment potential method.

We present a generalized Kohn-Sham (KS) density functional theory (DFT) based effective fragment potential (EFP2-DFT) method for the treatment of solvent effects. Similar to the original Hartree-Fock (HF) based potential with fitted parameters for water (EFP1) and the generalized HF based potential (EFP2-HF), EFP2-DFT includes electrostatic, exchange-repulsion, polarization, and dispersion potentials, which are generated for a chosen DFT functional for a given isolated molecule. The method does not have fitted parameters, except for implicit parameters within a chosen functional and the dispersion correction to the potential. The electrostatic potential is modeled with a multipolar expansion at each atomic center and bond midpoint using Stone's distributed multipolar analysis. The exchange-repulsion potential between two fragments is composed of the overlap and kinetic energy integrals and the nondiagonal KS matrices in the localized molecular orbital basis. The polarization potential is derived from the static molecular polarizability. The dispersion potential includes the intermolecular D3 dispersion correction of Grimme et al. [J. Chem. Phys. 132, 154104 (2010)]. The potential generated from the CAMB3LYP functional has mean unsigned errors (MUEs) with respect to results from coupled cluster singles, doubles, and perturbative triples with a complete basis set limit (CCSD(T)/CBS) extrapolation, of 1.7, 2.2, 2.0, and 0.5 kcal/mol, for the S22, water-benzene clusters, water clusters, and n-alkane dimers benchmark sets, respectively. The corresponding EFP2-HF errors for the respective benchmarks are 2.41, 3.1, 1.8, and 2.5 kcal/mol. Thus, the new EFP2-DFT-D3 method with the CAMB3LYP functional provides comparable or improved results at lower computational cost and, therefore, extends the range of applicability of EFP2 to larger system sizes.

A study of the reactivity of silver azide based on calculations of the band properties within the framework of density functional theory

Density functional theory methods in the basis of localized orbitals are used to study structural, electronic, vibrational, and thermodynamic properties, as well as the reactivity of the orthorhombic, tetragonal, and monoclinic phases of silver azide. The effects of pressure and temperature are described using the Debye quasi-harmonic model. The reactivity of silver azide is analyzed in terms of the enthalpy and entropy factors and equilibrium constants. It is shown that, for the solid-phase decomposition, a decrease in the volume (increase in pressure) leads to a decrease in the enthalpy, whereas the entropy factor remains virtually unchanged. As a result, at a pressure of 11.6 GPa, the reaction of direct generation of holes becomes feasible and then, above 14.8 GPa, exothermic. The formation of small nuclei of silver dinitride-nitride AgNN2 metastable phase is considered as a possible mechanism of generation of reaction kernels.

First principles calculation of elastic and magnetic properties of Cr-based full-Heusler alloys

We present an ab-initio study of the elastic and magnetic properties of Cr-based full-Heusler alloys within the first-principles density functional theory. The lattice constant, magnetic moment, bulk modulus and density of states are calculated using the full-potential nonorthogonal local-orbital minimum basis (FPLO) code in the Generalized Gradient Approximation (GGA) scheme. Only the two alloys Co2CrSi and Fe2CrSi are half-metallic with energy gaps of 0.88 and 0.55eV in the spin-down channel respectively. We have predicted the metallicity state for Fe2CrSb, Ni2CrIn, Cu2CrIn, and Cu2CrSi alloys. Fe2CrSb shows a strong pressure dependent, e.g. exhibits metallicity at zero pressure and turns into a half-metal at P≥10GPa. The total and partial magnetic moments of these alloys were studied under higher pressure, e.g. in Co2CrIn, the total magnetic moment is almost unchanged under higher pressure up to 500GPa.