Local Density Functional Calculations Research Articles

GPU acceleration of local and semilocal density functional calculations in the SPARC electronic structure code.

We present a Graphics Processing Unit (GPU)-accelerated version of the real-space SPARC electronic structure code for performing Kohn-Sham density functional theory calculations within the local density and generalized gradient approximations. In particular, we develop a modular math-kernel based implementation for NVIDIA architectures wherein the computationally expensive operations are carried out on the GPUs, with the remainder of the workload retained on the central processing units (CPUs). Using representative bulk and slab examples, we show that relative to CPU-only execution, GPUs enable speedups of up to 6× and 60× in node and core hours, respectively, bringing time to solution down to less than 30s for a metallic system with over 14 000 electrons and enabling significant reductions in computational resources required for a given wall time.

Weak ferromagnetism in hexagonal Mn3Z alloys (Z=Sn,Ge,Ga)

We present combined spin model and first principles electronic structure calculations to study the weak ferromagnetism in bulk Mn$_3$Z (Z=Sn, Ge, Ga) compounds. The spin model parameters were determined from a spin-cluster expansion technique based on the relativistic disordered local moment formalism implemented in the screened Korringa--Kohn--Rostoker method. We describe the magnetic ground state of the system within a three-sublattice model and investigate the formation of the weak ferromagnetic states in terms of the relevant model parameters. First, we give a group-theoretical argument how the point-group symmetry of the lattice leads to the formation of weak ferromagnetic states. Then we study the ground states of the classical spin model and derive analytical expressions for the weak ferromagnetic distortions by recovering the main results of the group-theoretical analysis. As a third approach we obtain the weak ferromagnetic ground states from self-consistent density functional calculations and compare our results with previous first principles calculations and with available experimental data. In particular, we demonstrate that the orbital moments follow a decomposition predicted by group theory. For a deeper understanding of the formation of weak ferromagnetism we selectively trace the effect of the spin-orbit coupling at the Mn and Z sites. In addition, for the case of Mn$_3$Ga, we gain information on the role of the induced moment of Ga from constrained local density functional calculations.

Interaction between Bi Dopants and Intrinsic Defects in LiNbO3 from Local and Hybrid Density Functional Theory Calculations.

The interactions between Bi dopants including Bi-substituting Li (BiLi) and Bi-substituting Nb (BiNb) and the intrinsic antisite defects (NbLi) and Li vacancies (VLi) in LiNbO3 are investigated using local and hybrid density functional theories. Three charge-compensated defect clusters, BiLi4+ + NbLi4+ + 8VLi-, BiLi4+ + 4VLi-, and BiLi4+ + BiNb0 + 4VLi-, are modeled in this work to investigate the effects of the Bi concentration. The most stable cluster configurations, the Bi-doping stability in the clusters, and the electronic state interaction between Bi and intrinsic defects have been studied in detail. It is found that BiLi4+ has a stronger electron-capturing ability than NbLi4+ in Bi-doped congruent LiNbO3. The BiLi-doping-induced local lattice distortion and the electron-trapping behavior remain unchanged with increasing Bi-doping concentration. However, the position of the Bi defect states in the band gap is found to be shifted in congruent LiNbO3. This is mainly attributed to the large lattice relaxation induced by the large number of Li vacancies instead of the ionic level redistribution caused by the direct interaction between Bi and intrinsic defects.

Effect of Hartree-Fock pseudopotentials on local density functional theory calculations.

Density functional theory (DFT) can run into serious difficulties with localized states in elements such as transition metals with occupied d states and oxygen. In contrast, including a fraction of the Hartree-Fock exchange can be a better approach for such localized states. Here, we develop Hartree-Fock pseudopotentials to be used alongside DFT for solids. The computational cost is on a par with standard DFT. Calculations for a range of II-VI, III-V and group-IV semiconductors with diverse physical properties show an observably improved band gap for systems containing d-electrons, pointing to a new direction in electronic theory.

Attosecond time delays in the valence photoionization of xenon and iodine at energies degenerate with core emissions

SynopsisWe perform time-dependent local density functional calculations of the quantum phase and Wigner-Smith time delays in the valence photoionization of Xe versus I atoms at higher energies where coupling with the core emission becomes important. Results for the 5s emission are presented.

Attosecond time delays in the photoionization of noble gas atoms studied in TDLDA

SynopsisWe perform time-dependent local density functional calculations of the quantum phase and time delays of valence photoionization of noble gas atoms. Results may be accessed by XUV-IR interferometric metrology.

Attosecond time delays in C60 valence photoemissions at the giant plasmon

SynopsisWe perform time-dependent local density functional calculations of the time delay in C60 HOMO and HOMO-1 photoionization at giant plasmon energies. A semiclassical model is used to develop further insights.

2D-GaS as a Photocatalyst for Water Splitting to Produce H₂.

Using first-principles local and hybrid density functional theoretical calculations, a thickness-dependent electronic structure of layered GaS is determined, and it is shown that 2D GaS has an electronic structure with valence and conduction bands that straddle the redox potentials of hydrogen evolution reaction and oxygen evolution reaction up to a critical thickness (<5.5 nm). Here, simulations of adsorption of H2O on nanoscale GaS reveal that localized electronic states at its edges appear in the gap and strengthen the interaction with H2O, further activating the surface atomic sites. It is thus predicted that GaS synthesized with a controlled thickness and preferred edges may be an efficient catalyst for photocatalytic splitting of water. Experiments that verify some of the predictions in this study are presented, and it is shown that GaS is effective in absorption of light and evolution of H2 (887 μmol h(-1) g(-1)) in the presence of aqueous solution of hydrazine (1% v/v). This study should open up the use of nanoscale GaS in conversion of solar energy into environment-friendly chemical energy in the form of hydrogen.

Strain effects on the band gap and work function of zigzag single-walled carbon nanotubes and graphene: A comparative study

First-principles local density functional calculation of the electronic structure and work function of zigzag single-wall carbon nanotubes (SWCNTs) and graphene under strain are presented. We found that there is a small circumference strain even for unstrained zigzag SWCNTs that release the curvature energy of the tube. Therefore, we propose that there are two effects contributing to the opening of the band gap of unstrained (3q, 0) SWCNTs: the electron transfer enhancement effect and the circumference strain effect. We show that the band gap and work function of strained zigzag SWCNTs can be successfully estimated from the band structure and work function of strained graphene based on the zone folding method with circumference strain effect. The circumference strain effect is crucial to obtain a correct result. The rate of change of the band gap with respect to the strain for zigzag SWCNTs is almost independent of the value of the strain and the radius of the tube.

Tuning from Half-Metallic to Semiconducting Behavior in SiC Nanoribbons

Half-metallic nanoscale conductors, highly sought after for spintronic applications, are usually realized through metal elements, chemical doping, or external electric fields. By means of local and hybrid density functional theory calculations, we identify pristine zigzag silicon carbide nanoribbons (zSiC-NRs) with bare edges as a metal-free monolayered material that exhibits intrinsic half-metallic behavior without chemical doping or an external electric field. Ab initio molecular dynamics simulations indicate that the half-metallicity is robust at room temperature. We also demonstrate that edge termination with O and S atoms transforms the zSiC-NRs into a full metal or a semiconducting material, respectively, due to the presence of O dimerization only on the Si edge and of S trimerization on both Si and C edges, the latter being driven by an unusual Peierls-like distortion along the functionalizing S atoms. The rich electronic properties displayed by zSiC-NRs may open new perspectives for spintronic app...

Magnetic Moment Enhancement at Transition Metal and Rare-Earth Metal Surfaces

Magnetism at surface represents an area of great interest in surface physics because, as expected, the lowered symmetry and coordination number offer a variety of opportunities for inducing new and exotic phenomena and so hold out the promise of new device applications. Here we review the prediction of local spin density functional ab initio electronic structure calculations which have played a key role in the development of this exciting field by providing a clearer understanding of the experimental observations and predicting new systems with desired properties.

Accurate evaluation of subband structure in a carrier accumulation layer at an n-type InAs surface: LDF calculation combined with high-resolution photoelectron spectroscopy

Adsorption on an n-type InAs surface often induces a gradual formation of a carrier-accumulation layer at the surface. By means of high-resolution photoelectron spectroscopy (PES), Betti et al. made a systematic observation of subbands in the accumulation layer in the formation process. Incorporating a highly nonparabolic (NP) dispersion of the conduction band into the local-density-functional (LDF) formalism, we examine the subband structure in the accumulation-layer formation process. Combining the LDF calculation with the PES experiment, we make an accurate evaluation of the accumulated-carrier density, the subband-edge energies, and the subband energy dispersion at each formation stage. Our theoretical calculation can reproduce the three observed subbands quantitatively. The subband dispersion, which deviates downward from that of the projected bulk conduction band with an increase in wave number, becomes significantly weaker in the formation process. Accurate evaluation of the NP subband dispersion at each formation stage is indispensable in making a quantitative analysis of collective electronic excitations and transport properties in the subbands.

P‐doping of Si nanoparticles: The effect of oxidation

AbstractThe radial dependence of the formation energy of substitutional phosphorus in silicon nanoparticles covered by an amorphous oxide shell is analysed using local density functional theory calculations. It is found that P+ is more stable at the silicon core. This explains the experimental observation of segregation of phosphorus to the Si‐rich regions in a material consisting of Si nanocrystals embedded in a SiO2 matrix [Perego et al., Nanotechnology 21, 025602 (2010)]. magnified imageFormation energy of positively charged substitutional phosphorus in a 1.5 nm diameter Si nanoparticle covered by a ∼2 nm‐thick amorphous SiO2 shell, as a function of its distance to the centre.

Multiscale Modeling of Reinforced Epoxy Resins by Carbon Nanotubes and Graphene

ABSTRACTNanocomposites are of increasing interest due to their unique structural, electronic, and thermal properties. Simultaneously, multiscale molecular modeling is becoming more robust. Therefore computational models are able to be examined with increased accuracy, complexity, and dimension. Graphene based molecules are lauded for their conductive properties as well as their architecture-like geometry which may allow bottom up nanoscale fabrication of nanoscopic structures. Furthermore, these macrocycled molecules allow high interactivity with other molecules including highly tensiled polymers that yield other novel supramolecular structures when interacted. These supramolecular structures are being investigated in lieu of a variety of potential applications. Nanocomposites of cured epoxy resin reinforced by single-walled carbon nanotubes exhibit a plethora of interesting behavior at the molecular level. A fundamental issue is how the self-organized dynamic structure of functional molecular systems affects the interactions of the nano-reinforced composites. A combination of force-field based molecular dynamics and local density-functional calculations shows that the stacking between the aromatic macrocycle and the surface of the SWNTs manifests itself via increased interfacial binding. First-principles calculations on the electronic structures further reveal that there exists distinct level hybridization behavior for metallic and semiconducting nanotubes. In addition there is a monatomic increase in binding energy with an increase in the nanotube diameter. The simulation studies suggest that graphene nanoplatelets are potentially the best fillers of epoxy matrices. The implications of these results for understanding dispersion mechanism and future nanocomposite developments are discussed.

Li-related defects in ZnO: Hybrid functional calculations

Hybrid density functional calculations are used to model substitutional lithium (LiZn) and interstitial lithium (Lii) defects in ZnO. The calculated transition levels and formation energies are compared to those obtained using a semilocal density functional scheme which lead to a severe band gap underestimation. It is found that the acceptor level of LiZn is deeper than has been predicted by local density functional calculations. Further, we compare the defect formation energies obtained using the two methodologies. It is shown that Lii+ is the dominant form in Zn-rich and O-rich p-type material, but LiZn- is still more stable than Lii in O-rich n-type material.

Bulk Modulus and Electronic Band Structure of ZnGa2X4 (X = S,Se): a First-Principles Study

First-principles local density functional calculations are presented for the compounds ZnGa2X4 (X = S,Se). We investigate the bulk moduli and electronic band structures in a defect chalcopyrite structure. The lattice constants and internal parameters are optimized. The electronic structures are analysed with the help of total and partial density of states. The relation between the cohesive energy and the unit cell volume is obtained by fully relaxed structures. We derive the bulk modulus of ZnGa2X4 by fitting the Birch–Murnaghan's equation of state. The extended Cohen's empirical formula agrees well with our ab initio results.

Accurate local density functional calculations with relativistic two-spinor minimax and finite element method for the alkali dimers

The 2-spinor minimax method combined with the numerical finite element method has proved to be very accurate. One obtains benchmark values in relativistic density functional calculations for diatomic molecules, with the error decreasing systematically with basis set size. In particular, the relativistic contamination errors and the variational collapse, which may occur in the relativistic 4-spinor calculations, are avoided in the 2-spinor minimax formulation of the Dirac equation by exact projection against the negative continuum. In the present work, we investigate the relativistic and nonrelativistic local-density functionals on alkali dimers (Li2 through Fr2), presenting accurate results in the sense that we have an optimized error control. We show that our results have minimal solution errors and report the most accurate values obtained for the respective functional, thus exhibiting the correct behaviour of these functionals and corresponding trends with increasing the atomic charge Z. In comparison with experiment the alkali dimers exhibit different behaviour than the dimers of group 11, the noble metal group including roentgenium (Rg, Z = 111), which were investigated in our previous papers. This is a clear indication that the correlation functional is important for the proper description of alkali dimers.

FP-LMTO investigations of mechanical stability and high pressure of platinum nitride compounds

We report local density functional calculations using the full potential linear muffin-tin orbital (FP-LMTO) method for binary platinum nitride (PtN), in five different crystal structures, the rock salt (B1), zinc-blende (B3), wurtzite (B4), nickel arsenide (B8), and PbS (B10) phases. The ground state properties such as the equilibrium lattice constant, elastic constants, the bulk modulus and its pressure derivative of PtN in these phases are determined and compared with the other available experimental and theoretical works. Our calculations confirm in the B3 structure that PtN is found to be mechanically stable with a large bulk modulus B=232.45 GPa and at a sufficiently high pressure the B8 1 structure would be favoured. The theoretical transition pressure from zinc blende (B3) to NiAs (B8 1), zinc-blende (B3) to rock-salt (B1) and zinc-blende (B3) to PbO (B10) is determined to be 9.10 GPa, 9.85 GPa and 69.35 GPa, respectively. Our calculation shows also in five different structures for PtN a high bulk modulus is a good indicator of a hard material.

Energy gap opening in submonolayer lithium on graphene: Local density functional and tight-binding calculations

The adsorption of an alkali-metal submonolayer on graphene occupying every third hexagon of the honeycomb lattice in a commensurate $(\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3})R30\ifmmode^\circ\else\textdegree\fi{}$ arrangement induces an energy gap in the spectrum of graphene. To exemplify this type of band gap, we present ab initio density functional theory calculations of the electronic band structure of ${\text{C}}_{6}\text{Li}$. An examination of the lattice geometry of the compound system shows the possibility that the nearest-neighbor hopping amplitudes have alternating values constructed in a Kekul\'e-type structure. The band structure of the textured tight-binding model is calculated and shown to reproduce the expected band gap as well as other characteristic degeneracy removals in the spectrum of graphene induced by lithium adsorption. More generally we also deduce the possibility of energy gap opening in periodic metal on graphene compounds ${\text{C}}_{x}M$ if $x$ is a multiple of 3.

Bis(terpyridine)-based surface template structures on graphite: a force field and DFT study

Host-guest networks formed by ordered organic layers are promising candidates for applications in molecular storage and quantum computing. We have studied 2-dimensionally ordered surface template structures of bis(terpyridine)-derived molecules (BTPs) on graphite using force field and DFT methods and compared the results to recent experimental observations. In order to determine the force field best suited for surface calculations, bond lengths and angles, torsional potentials, adsorption and stacking energies of smaller aromatic molecules were calculated with different force fields (Compass, UFF, Dreiding and CVFF). Density functional perturbation theory calculations were used to study the intermolecular interactions between 3,3'-BTP molecules. Structural properties, adsorption energies and rotational barriers of the 3,3'-BTP surface structure and its host-guest systems with phthalocyanine (PcH(2)) or excess 3,3'-BTP as guest molecules have been addressed. In addition, STM images of oligopyridine and phthalocyanine molecules were simulated based on periodic and local density functional theory calculations.