Ab Initio Pseudopotentials Research Articles

Density functional simulation of water molecule with Ab-initio pseudo potential

Density functional simulation of water molecule with Ab-initio pseudo potential

Soft and transferable pseudopotentials from multi-objective optimization

Ab initio pseudopotentials are a linchpin of modern molecular and condensed matter electronic structure calculations. In this work, we employ multi-objective optimization to maximize pseudopotential softness while maintaining high accuracy and transferability. To accomplish this, we develop a formulation in which softness and accuracy are simultaneously maximized, with accuracy determined by the ability to reproduce all-electron energy differences between Bravais lattice structures, whereupon the resulting Pareto frontier is scanned for the softest pseudopotential that provides the desired accuracy in established transferability tests. We employ an evolutionary algorithm to solve the multi-objective optimization problem and apply it to generate a comprehensive table of optimized norm-conserving Vanderbilt (ONCV) pseudopotentials (https://github.com/SPARC-X/SPMS-psps). We show that the resulting table is softer than existing tables of comparable accuracy, while more accurate than tables of comparable softness. The potentials thus afford the possibility to speed up calculations in a broad range of applications areas while maintaining high accuracy.

Ab initio studies on a highly luminescent and oxygen-sensitive Tb(III) complex with a 1,4,7-triazacyclononane-based tris-aryloxide ligand

The strongly luminescent and highly oxygen-sensitive Tb(III) complex [{(MeMeArO)3tacn}TbIII(THF)] (denoted hereafter as [1Tb THF]) was studied with density functional theory (DFT) and wavefunction-based ab initio electronic structure methods combined with Tb relativistic ab initio pseudopotentials. The optimized geometries as well as the calculated UV–vis absorption spectra at the DFT level agree well with the available experimental data. The calculated vertical S0*/5D4 → S0*/7FJ (ligand/Tb3+ states) emission energies for J = 0–6 at the RSPT2//CASSCF level including spin-orbit corrections are 475, 511, 549, 583, 614, 638, and 644 nm, respectively, and are lower by 15, 36, 39, 39, 39, 35, and 35 nm, respectively, than the corresponding experimental values. The luminescence quenching mechanism of [1Tb THF] in presence of O2 was investigated. RSPT2//CASSCF calculations for [1Tb THF]·O2 indicate that the S0*/7FJ (J = 0–6)/1O2 (1Δg or 1Σ+g) states (ligand/Tb3+/O2 states) are energetically more stable than the emission states S0*/5DJ (J = 0–4)/3O2 (3Σ-g). Therefore, after initial photo excitation of the ligand and subsequent energy transfer to the Tb3+ ion, the energy will be further transferred to O2 leading to the observed luminescence quenching of [1Tb THF] in air. A comparison is made to [1Sm THF]·O2 which shows a significantly smaller luminescence quantum yield and no oxygen quenching.

Density functional theory method for twisted geometries with application to torsional deformations in group-IV nanotubes

We present a real-space formulation and implementation of Kohn-Sham Density Functional Theory suited to twisted geometries, and apply it to the study of torsional deformations of X (X = C, Si, Ge, Sn) nanotubes. Our formulation is based on higher order finite difference discretization in helical coordinates, uses ab initio pseudopotentials, and naturally incorporates rotational (cyclic) and screw operation (i.e., helical) symmetries. We discuss several aspects of the computational method, including the form of the governing equations, d ils of the numerical implementation, as well as its convergence, accuracy and efficiency properties.The technique presented here is particularly well suited to the first principles simulation of quasi-one-dimensional structures and their deformations, and many systems of interest can be investigated using small simulation cells containing just a few atoms. We apply the method to systematically study the properties of single-wall zigzag and armchair group-IV nanotubes in the range of (approximately) 1 to 3 nm radius, as they undergo twisting. For the range of deformations considered, the mechanical behavior of the tubes is found to be largely consistent with isotropic linear elasticity, with the torsional stiffness, ktwist, varying as the cube of the nanotube radius. Furthermore, for a given tube radius, ktwist is seen to be highest for carbon nanotubes and the lowest for those of tin, while nanotubes of silicon and germanium are found to have intermediate values of this quantity close to each other. We also describe different aspects of the variation in electronic properties of the nanotubes as they are twisted. In particular, we find that akin to the well known behavior of armchair carbon nanotubes, armchair nanotubes of silicon, germanium and tin also exhibit bandgaps that vary periodically with imposed rate of twist, and that the periodicity of the variation scales in an inverse quadratic manner with the tube radius. These examples highlight the utility of the proposed method in the accurate and efficient computational characterization of important nanomaterials from first principles.

Evaluating Hellmann–Feynman forces within non-local pseudopotentials

A new approach for evaluating Hellmann–Feynman forces within a non-local potential is introduced. Particularly, the case of Hellmann–Feynman theorem applied within density functional theory in combination with nonlocal ab-initio pseudopotentials, discretized by the finite-element method, is discussed in detail. The validity of the new approach is verified using test calculations on simple molecules and the convergence properties (w.r.t. the DFT loop) are analyzed. A comparison to other previously published approaches to Hellmann–Feynman forces calculations is shown to document that the new approach mitigates, for l-dependent as well as for separable forms of nonlocal pseudopotentials, the efficiency and/or accuracy problems arising in the methods published so far.

Hydrogen Effect on Electron-Phonon Interactions in L10 FePd

The effect of H position in L10FePd magnetic equiatomic layered structure on electron-phonon interactions is studied via ab initio pseudopotentials density functional approach in the general gradient approximation. The structures investigated are L10FePd (parent structure), and the specific hydrides 2(FePd)1H (H@Fe layer) and 2(FePd)2H (H@Pd layer). At ground state (0 K), electronic band structure results demonstrate the emergence of Fermi surfaces (FS) in topologies of small isolated multipockets in the hydrides. Using the harmonic approximation I find the phonon energy dispersion of parent and H@Fe simple and classical but for H@Pd complex. I linked this latter structure to its topology of FS, known as Kohn anomaly. Using McMillan-Eliashberg model, I get superconducting transition temperature (Tc) values ≈ 0.00 K, 3.48 K, and 19.75 K for the parent, H@Fe and H@Pd respectively. Therefore, these hypothetical hydrides prove that H position in the structure has a direct influence on Tc values and consequently the suppression of magnetism.

Simulating contrast inversion in atomic force microscopy imaging with real-space pseudopotentials

Atomic force microscopy (AFM) measurements have reported contrast inversions for systems such as ${\mathrm{Cu}}_{2}\mathrm{N}$ and graphene that can hamper image interpretation and characterization. Here, we apply a simulation method based on ab initio real-space pseudopotentials to gain an understanding of the tip-sample interactions that influence the inversion. We find that chemically reactive tips induce an attractive binding force that results in the contrast inversion. We find that the inversion is tip height dependent and not observed when using less reactive CO-functionalized tips.

Repulsive tip tilting as the dominant mechanism for hydrogen bond-like features in atomic force microscopy imaging

Experimental atomic force microscopy (AFM) studies have reported distinct features in regions with little electron density for various organic systems. These unexpected features have been proposed to be a direct visualization of intermolecular hydrogen bonding. Here, we apply a computational method using ab initio real-space pseudopotentials along with a scheme to account for tip tilting to simulate AFM images of the 8-hydroxyquinoline dimer and related systems to develop an understanding of the imaging mechanism for hydrogen bonds. We find that contrast for the observed “hydrogen bond” feature comes not from the electrostatic character of the bonds themselves but rather from repulsive tip tilting induced by neighboring electron-rich atoms.

Structural, electronic and optical properties of Li intercalated on MO3 (M: Mo, W): A first principle investigation

Abstract The correlations between electronic and optical properties of pure MO 3 (M=Mo, W) and the effect of Li intercalated in the matrix are investigated within density functional theory using both the revised gradient-corrected Perdew–Burke–Ernzerhof (GGA-PBE), GGA+U and HSE06 functional frameworks along with ab initio pseudopotentials method. Calculation performed with HSE06 functional proves that all these compounds are wide band gap semiconductors. The energy gap value is found to decrease with lithium incorporation, affecting the visible absorption coefficients.

Hydrogen in palladium: Anharmonicity of lattice dynamics from first principles

The interaction potentials of the palladium and hydrogen sublattices at different hydrogen concentrations have been obtained in terms of the density functional theory and ab initio pseudopotentials. It has been shown that the anharmonicity of this interaction depends on the hydrogen concentration. The phonon spectrum of palladium hydride PdH has been calculated in the harmonic approximation and taking into account the anharmonic effects. The temperature-dependent effective potential technique accounting for the anharmonic effects of lattice vibrations has been described.

First-principles study of electronic and dynamical properties of the TaC(001) surface

We have made theoretical investigations of the atomic geometry, electronic structure, and lattice dynamics of the (001) surface of TaC. The atomic geometry and electronic structure for this surface have been calculated by using the generalised gradient approximation of the density functional theory and ab initio pseudopotentials. Lattice dynamical results are obtained by employing the density functional perturbation method. At least three surface phonon states appear throughout the surface Brillouin zone in the acoustic–optical gap range. Experimentally identified surface mode results are reproduced and their origin explained.

Polytypism of GaAs, InP, InAs, and InSb: Anab initiostudy

A systematic study of ground state properties of cubic (3$C$) and hexagonal (6$H$, 4$H$, 2$H$) polytypes of normal (nonnitride) III-V compounds is reported using well-converged density-functional calculations within local density approximation, ab initio pseudopotentials, and projector-augmented wave method. Equilibrium results are obtained for lattice parameters, cohesive energies, and bulk moduli. Internal degrees of freedom, i.e., atomic relaxations, are taken into account. Trends with the hexagonality are discussed. Within the axial next-nearest-neighbor Ising (ANNNI) model driving forces of the polytypism as well as the stacking fault formation are derived. The results are compared with available experimental data.

Ab initio calculation of the thermodynamic properties of liquid alkali metals

The original ab initio pseudopotential that is based on the orthogonalized plane wave method and was proposed earlier is applied for the calculation of the thermodynamic properties of liquid metals in combination with a variational approach. Numerical results are obtained for pure Li, Na, and K and an Na-K alloy. The calculated results are compared with the experimental data, and the possible causes of the discrepancy are described.

Ab initiostudy of electron paramagnetic resonance hyperfine structure of the silicon dangling bond: Role of the local environment

The hyperfine coupling of the electron spin to the nuclear spins nearby contains information on the local environment such as the chemical composition, distances, or bond angles. A correct interpretation of this information requires a comparison to a theoretical model. In this work, we employ spin-density-functional theory and ab initio pseudopotentials to study how the hyperfine couplings of a Si dangling bond change under systematic variations of the local environment. For our network models, which take the effect of the extended host into account (supercell approach), we show that the hyperfine tensor of the undercoordinated silicon atom is governed by the interplay between $s\ensuremath{-}p$ hybridization and the degree of localization of the defect state.

Towards the Reduction of Optical Losses in Transition Metal-Based Nanomaterials

ABSTRACTEquilibrium geometries and cohesion energies of Ag0.94Cd0.06, Ag0.94In0.06, Au0.94Cd0.06, and Au0.94In0.06 solid alloys have been studied from the first principles within the Density Functional Theory using ab initio pseudopotentials. Equilibrium geometries are obtained by total energy minimization method using the Local Density Approximation and Generalized Gradient Approximation methods. Optical functions are calculated within the independent particles picture. We report essentially different behavior of Cd and In impurity atoms in Au- and Ag-based alloys: the aggregated (or quasi aggregated) phases in In-containing alloys are expected in contrast to the alloys with Cd atom where homogeneous impurity distribution over the bulk should dominate. Study of optical spectra in Ag0.94Cd0.06 and Au0.94Cd0.06 alloys indicate that optical losses in visible and near ultraviolet spectral range remarkably increase at bigger Cd concentrations. In ultraviolet spectral region redistribution of optical oscillator strengths results in both increase and decrease of optical losses in selected spectral regions.

Influence of the interface atomic structure on the magnetic and electronic properties ofLa2/3Sr1/3MnO3/SrTiO3(001)heterojunctions

The magnetic and electronic properties of abrupt ${\text{La}}_{2/3}{\text{Sr}}_{1/3}{\text{MnO}}_{3}/{\text{SrTiO}}_{3}(001)$ heterojunctions are investigated by means of generalized gradient approximation (GGA) and $\text{GGA}+U$ calculations based on ab initio pseudopotentials. We address, in particular, the influence of the ${\text{TiO}}_{2}(001)$- and SrO(001)-layer termination of the ${\text{SrTiO}}_{3}$ at the interface on the magnetic structure, Schottky barrier height, and existence of localized interface states in these junctions. Both interface terminations are found to lead, at zero temperature, to a ferromagnetic coupling between the magnetic moments of the Mn at the junction and in bulk ${\text{La}}_{2/3}{\text{Sr}}_{1/3}{\text{MnO}}_{3}$. The energy difference, however, between configurations with antiferromagnetic and ferromagnetic alignments at the junction is small in the case of the SrO termination, suggesting that configurations with swapped Mn spins at the interface are likely to occur at ambient temperature. The interface termination has a major influence on the Schottky barrier height. The $p$-type Schottky barrier height is increased at the abrupt ${\text{TiO}}_{2}$-terminated interface, relative to the abrupt SrO-terminated junction. Although no minority-spin interface state occurs near the Fermi energy in the abrupt defect-free ${\text{La}}_{2/3}{\text{Sr}}_{1/3}{\text{MnO}}_{3}/{\text{SrTiO}}_{3}(001)$ heterojunctions, low-formation-energy magnetic defects corresponding to swapped Mn spins at the SrO-terminated interface produce localized interface states at the Fermi energy. Such states may jeopardize the spintronic properties of ${\text{La}}_{2/3}{\text{Sr}}_{1/3}{\text{MnO}}_{3}/{\text{SrTiO}}_{3}$-based structures near room temperature.

Ab initio lattice dynamics of CoH and NiH

Ground state parameters and lattice dynamics of CoH and NiH phases are studied by means of the state-of-the-art ab initio pseudopotentials methods. Calculated ground state parameters and phonon DOS are in rather good agreement with available experimental data. No phase transformation for CoH and NiH was found in a wide pressure and temperature range.

ChemInform Abstract: Pseudopotential Study on Rare Earth Dihalides and Trihalides

Abstract Self-consistent-field (SCF) calculations and configuration interaction calculations including single and double substitutions (CISD) using one-component quasi-relativistic ab initio pseudopotentials were performed for selected dihalides and trihalides of the rare earth elements. The results are compared with earlier theoretical investigations and with available experimental data. The experimentally observed trends in bond lengths, bond angles and atomization energies are well reproduced. Estimates are given for experimentally unknown data.

Hydrogen in the approximant i-TiZrHf: The energy state, charge, and diffusion

The energy of hydrogen dissolution in different tetrahedral pores of a 1/1 approximant of the icosahedral TiZrHf quasicrystal has been determined in terms of the density functional theory and ab initio pseudopotentials. At low and high degrees of loading of TiZrHf with hydrogen, the charges of atoms involved in the system and the Mayer bond order have been calculated for all possible M-M and M-H pairs. The diffusion coefficient of a single hydrogen atom in the system under study has been estimated numerically.

Pseudopotential calculations. IV. Some results for zinc difluoride

Recently developed ab initio pseudopotentials are employed to examine the molecular electronic structure of ZnF/sub 2/ in an LCAO-HF treatment. Comparison with a previous all-electron study reveals excellent agreement. Comparison with a pseudopotential treatment of ZnCl/sub 2/ shows, as expected, that the fluoride is far more ionic than the chloride; this seems to arise largely from more halide participation in the a/sub 2u/ nonbonding orbital.