Perdew-Wang Generalized Gradient Approximation Research Articles

Influence of Te doping in titanium dichalcogenides: LCAO calculations and Compton spectroscopy

Role of Te doping in titanium disulfide using linear combination of atomic orbitals method and Compton spectroscopy is reported. The theoretical Compton profiles (CPs) of TiSTe derived using various exchange and correlation potentials like Perdew-Burke-Ernzerhof, Perdew-Wang generalized gradient approximation and von Barth-Hedin (VBH) are compared with corresponding experimental CP to check the applicability of considered potentials. From the total and partial density of states, we observe that doping of Te at S site vanishes the narrow band gap of TiS2 leading to metallic character of TiSTe. A good agreement between experimental and VBH potential based CP of TiSTe shows better performance of VBH approximation in this mixed metal-like dichalcogenide.

Quantum Calculation for Musk Molecules Infrared Spectra towards the Understanding of Odor

It is not clear so far how humans can recognize odor. One of the theories regarding structure-odor relationship is vibrational theory, which claims that odors can be recognized by their modes of vibration. In this sense, this paper brings a novel comparison made between musky and nonmusky molecules, as to check the existence of correlation between their modes on the infrared spectra and odor. For this purpose, sixteen musky odorants were chosen, as well as seven other molecules that are structurally similar to them, but with no musk odor. All of them were submitted to solid theoretical methodology (using molecular mechanics/molecular dynamics and Neglect of Diatomic Differential Overlap Austin Model 1 methods to optimize geometries) as to achieve density functional theory spectra information, with both Gradient Corrected Functional Perdew-Wang generalized-gradient approximation (GGA/PW91) and hybrid Becke, three-parameter, Lee-Yang-Parr (B3LYP) functional. For a proper analysis over spectral data, a mathematical method was designed, generating weighted averages for theoretical frequencies and computing deviations from these averages. It was then devised that musky odorants satisfied demands of the vibrational theory, while nonmusk compounds belonging either to nitro group or to acyclic group failed to fulfill the same criteria.

Approaching chemical accuracy with density functional calculations: Diatomic energy corrections

Density functional theory (DFT) is widely used to predict materials properties, but the local density approximation (LDA) and generalized gradient approximation (GGA) exchange-correlation functionals are known to poorly predict the energetics of reactions involving molecular species. In this paper, we obtain corrections for the O${}_{2}$, H${}_{2}$, N${}_{2}$, F${}_{2}$, and Cl${}_{2}$ molecules within the Perdew-Burke-Enzerhof GGA, Perdew-Wang GGA, and Perdew-Zunger LDA exchange-correlation functionals by comparing DFT-calculated formation energies of oxides, hydrides, nitrides, fluorides, and chlorides to experimental values. We also show that the choice of compounds used to obtain the correction is significant, and we use a leave-one-out cross-validation approach to rigorously determine the proper fit set. We report confidence intervals with our correction values, which quantifies the variation caused by the choice of fit set after outlier removal. The remaining variation in the correction values is of the order of 1 kcal/mol, which indicates that chemical accuracy is a realistic goal for these systems.

Geometries and stabilities of Ag n v (v = ±1, 0; n = 21–29) clusters

We performed a systematical study on the lowest-energy structures of the medium-sized silver clusters Ag n (n = 21–29) by using a genetic algorithm coupled with a tight-binding method, and the DFT calculations with Perdew–Wang generalized-gradient approximation. The corresponding cluster ions were also searched based on the neutral cluster structures. It is found that the Ag21–23 prefer icosahedron or double-icosahedron as core structures. Ag n (n = 24–27) favor a bulk-like fcc stacking motif. Ag28 and Ag29 tend to high symmetrical structures. The relative stabilities, the ionization potentials and electronic affinities of silver clusters analyzed in the paper are consistent with the experimental data. It is interesting to find that the experimental spectra fit reasonable well the optical absorption spectra obtained with the structures calculated by us.

Density functional theory study on two-peak emission of Eu 2+ activators in Sr 2SiO 4

The supercells of pure and Eu-doped Sr 2SiO 4 were theoretically analyzed by density functional theory (DFT) calculations to investigate the typical two-peak emission of Sr 2SiO 4:Eu 2+, which originates from two different Sr 2+ (or Eu 2+) sites in the Sr 2SiO 4 host structure. The Perdew–Wang generalized-gradient approximation (GGA) functional and the double numerical plus d-functions (DND) basis set with effective core potentials (ECP) were employed in the calculations of electronic properties. The electron transfer between Eu 2+ ions placed at two different crystallographic Sr 2+ sites was understood based on the accurate assignment of deconvoluted peaks of the two-peak emission to their corresponding crystallographic sites. This study ought to be instructive as a basic guideline to improve the color chromaticity of Sr 2SiO 4:Eu 2+ for use in white light emitting diodes (WLEDs).

First principle investigation of AlAs and AlP compounds and ordered AlAs 1−xP x alloys

We have investigated the structural and electronic properties of AlAs and AlP compounds and of ordered AlAs 1− x P x alloys using the full potential linearized augmented plane wave plus local orbitals (FP-LAPW + lo) method based on density functional theory. The total energies and structural quantities of those compounds have been calculated for different approximations of exchange–correlation energy. The electronic quantities have been found to be in good agreement with the corresponding measured ones when the compounds were defined by the lattice constants of Perdew–Wang-generalized gradient approximation (PW-GGA) scheme. The PW-GGA approach was also applied on ordered AlAs 1− x P x alloys to study the effect of composition on lattice constant, band gap, and refractive index of AlAs 1− x P x ternary alloys. The calculated lattice constants scale linearly with composition (Vegard’s law). The microscopic origins of the gap bowing were explained by using the approach of Zunger and co-workers. In addition to (FP-LAPW + lo) method, the composition dependence of the refractive index was studied by Reedy and Nazeer model. The thermodynamic stability of these alloys was investigated by calculating the excess enthalpy of mixing Δ H m as well as the phase diagram.

First-principles analysis of the adsorption of aluminum and chromium atoms on the HfC (001) surface

Abstract We have investigated the surface energies and electronic structures of the HfC (0 0 1) (1 × 1) surface. Calculations were also carried out for the adsorption of Al and Cr atoms on the surface. The structural and electronic properties of bulk HfC were also determined. All of these calculations were based on first principles. We have dealt with the exchange and correlation potential for bulk HfC using the Perdew–Wang generalized gradient approximation (GGA-PW91) (1991). Furthermore, the local density approximation (LDA) was used for the adsorption energy calculations of Al and Cr atoms on the HfC (0 0 1) surface. For Al, Cr atoms on the surface, the adsorption energies confirm the preference for adsorption at the on-top C atoms sites. Moreover, compared with Al atoms, Cr atoms are more easily adsorbed on the HfC (0 0 1) surface. The Cr adatom–surface bonds are stronger than the Al adatom–surface bonds. The C–Al bonds are ionic while the C–Cr bonds present the covalent and metallic characteristics in nature.

Electron momentum distribution in SnS

AbstractA study of electron momentum density distribution in SnS by means of Compton profiles is presented. The measurement on polycrystalline sample of SnS is performed employing 59.54 keV gamma‐rays emanating from an Am241 radioisotope. The calculations are performed on the basis of density‐functional theory based ab‐initio linear combination of atomic orbitals method embodied in the CRYSTAL program. The correlation scheme proposed by von Barth and Hedin is adopted. The generalized gradient approximation proposed by Perdew and Wang is also considered to treat the correlation. The exchange is treated following the Becke scheme. Both the correlation schemes are in good agreement with the measurement. The better agreement with experimental result is, however, observed with the von Barth and Hedin correlation that also gives lower ground‐state energy than the Perdew–Wang generalized gradient approximation. Ionic model calculations for a number of configurations of Sn+xS−x (0.0 ≤ x ≤ 2) are also performed utilizing free‐atom profiles. The ionic model suggests transfer of 2.0 electrons from the valence 5p state of Sn to the 3p state of S.

Hybrid density functional theory applied to magnetite: Crystal structure, charge order, and phonons

The electronic structure and equilibrium structure of magnetite $({\text{Fe}}_{3}{\text{O}}_{4})$ in the high temperature cubic $Fd\overline{3}m$ and low temperature monoclinic $P2/c$ unit cells have been computed using the Perdew-Wang generalized gradient approximation (GGA) to density functional theory (DFT) and the B3LYP hybrid density functional. The ground state for the GGA-DFT is an itinerant electron metallic state in the cubic unit cell and the ground state for the B3LYP functional is a charge ordered semiconducting state in the monoclinic unit cell. The equilibrium structure predicted by the B3LYP functional for ${\text{Fe}}_{3}{\text{O}}_{4}$ in the $P2/c$ unit cell has been calculated with lattice parameters fixed at values obtained in recent x-ray diffraction work and with the lattice fully relaxed. Bond lengths obtained with lattice parameters fixed at experimental values are in excellent agreement with x-ray measurements [J. P. Wright et al., Phys. Rev. B 66, 214422 (2002)]. The degree of charge order, measured as disproportionation of charge on octahedral B sites, is considerably less than unity and in reasonable agreement with values from resonant x-ray diffraction measurements. However, conduction electrons are found to be fully localized on B1 and B4 sites in orbitally ordered ${t}_{2g}$ states. This shows that they are formally ${\text{Fe}}^{2+}$ ions while Fe B2 and B3 sites are formally ${\text{Fe}}^{3+}$ sites. Therefore Verwey's original conjecture regarding charge localization in ${\text{Fe}}_{3}{\text{O}}_{4}$ applies, even though the specific pattern of charge order is different. GGA-DFT and B3LYP density functionals were used to calculate phonons at the $\ensuremath{\Gamma}$ point of the Brillouin zone. Phonon frequencies predicted for these crystal structures are compared to frequencies from infrared conductivity and Raman scattering experiments. Charge ordering causes symmetry breaking of force constants on symmetry lowering from the cubic $Fd\overline{3}m$ unit cell to the $P2/c$ unit cell. This produces frequency splitting of modes which are degenerate in the cubic unit cell and concentration of ion displacements in phonon eigenvectors on particular Fe octahedral B site chains, especially in the highest frequency bands.

The structural and electronic properties of BN and BP compounds and BN xP 1− x alloys

The structural and electronic properties of BN and BP compounds and BN x P 1− x alloys have been investigated by full potential linearized augmented plane wave (FP-LAPW) method based on density functional theory (DFT). The total energies and electronic band structures of these compounds have been calculated for different approximations of exchange–correlation energy. The comparative study has showed that the Perdew–Wang-generalized gradient approximation (PW-GGA) is the best one to produce the measured structural quantities of the compounds, such as the lattice constant, bulk modulus, and first-order pressure derivative of the bulk modulus. The electronic quantities of the compounds, such as the width of the valance band and energy gaps at high symmetry points have been found to be in good agreement with the corresponding measured ones when the compounds were defined by the lattice constants of PW-GGA scheme. The PW-GGA approach was also applied on BN x P 1− x alloys for obtaining the variation of the equilibrium lattice constants, bulk moduli, and minimum energy gaps as a function of the nitrogen concentration within the range of 0 < x < 1 . It is found that these variations having large bowing parameters exclude the Vegard’s linear rule.

Electronic and structural properties of MgB2 by the linear combination of atomic orbitals method

In this paper theoretical calculations of electronic and structural properties, namely Compton profiles and structure factors of MgB2, are presented. The calculations are performed using periodic linear combination of atomic orbitals method. Exchange and correlation has been treated under the Perdew–Wang generalized gradient approximation (PW-GGA) and the hybrid Becke-3-Lee-Yang-Parr (B3LYP) schemes. The Compton profiles at 15, 55, and 293 K are calculated by considering only the thermal expansion of the lattice keeping all other computational parameters identical. The calculated Compton profiles have been compared with the published experimental data measured using synchrotron radiation. The calculated profiles under the PW-GGA scheme are well in accordance with the measurements. The best agreement at the level of first and second order derivatives of Compton profiles has, however, been found with the B3LYP hybrid function. In the superconducting state the momentum density shows sharper changes than the normal state around Fermi momentum. The structure factors for a number of reflection lines have also been computed and compared with the published electron diffraction and the synchrotron x-ray powder diffraction data. The calculations are found to be in good agreement with the measurements. The calculation supports the charge transfer from Mg plane (001) to the (002) plane containing boron atoms, well in agreement with the prediction of the measurement.

X-ray diffraction and ab initio determinations of the structure of Rb 4CdCl 6

The structure of Rb 4CdCl 6 has been examined both at the experimental and at the ab initio levels. Experimentally, new X-ray diffraction measurements have been performed on a single crystal grown at room temperature from aqueous solution. The compound is confirmed to crystallize with a rhombohedral unit cell (space group R 3 ̄ c ) but with a distorsion from cubic symmetry noticeably smaller than in a previous study [Kristallografiya, 37 (1992) 815]. From the ab initio point of view, three distinct sets of first-principles density functional theory calculations (Perdew–Wang generalized-gradient-approximation (PWGGA), Perdew, Burke and Ernzerhof parametrization and Becke-Lyp exchange-correlation functions) have been carried out. These ab initio results corresponding to the ideal crystal (i.e. infinite, periodic in a definite phase and defect-free) are in correct agreement with our own experimental findings, especially for the distorsion. Nevertheless, due to the relative flatness of the ideal crystal potential energy surface with respect to distorsion around the minimum, it is quite plausible the two presently available X-ray diffraction analyses, made on two different real (imperfect) samples, are both correct even though they come out onto significantly different mean distorsion parameters. The shape, orientation and relative sizes of the thermal vibration ellipsoids are discussed in relation with relevant aspects of the calculated potential energy surface. The PWGGA wavefunction is used to plot valence electronic charge density maps. The comparison of the charge distribution in Rb 4CdCl 6 with that in Rb 2CdCl 4 is discussed in terms of compound structures.

Ga-doping effects on electronic and structural properties of wurtzite ZnO

The electronic properties of Ga-doped ZnO with wurtzite structures are studied using the frozen-core projector augmented wave method based upon the density functional theory with the Perdew-Wang generalized gradient approximation. Among a series of doped cases with atomic ratio of 1.85%, 2.78%, 6.25%, and 12.5%, the system with a 12.5% level of doping yields the lowest cohesive energy in the hexagonal structure. A sharp resonance attributed to the Ga 4s orbital shows up just below the low-lying Zn 3d valence bands. The width of this peak becomes broader as the amount of the Ga dopant is increased, reflecting an increased interaction with the 2p orbitals of the surrounding O atoms. The calculated result reproduces the pronounced Burstein–Moss shift and the shrinkage of the fundamental band gap. Furthermore, an increase of the Ga content leads to the amplification of the volume of the unit cell. The present work seems to indicate that, when doping is introduced, the effective mass of the electron in conduction bands becomes slightly smaller than that in pure ZnO.

Density-functional calculations of semiconductor properties using a semiempirical exchange-correlation functional

We investigate the applicability of the semiempirical Hamprecht-Cohen-Tozer-Handy (HCTH) exchange-correlation functional, which was optimized by fitting to molecular systems, to the calculation of semiconductor lattice constants, bulk moduli, cohesive energies, and electronic band gaps. Plane-wave pseudopotential calculations are performed on a series of diamond and zinc-blende semiconductors and HCTH values are compared with the local density approximation (LDA) and the Perdew-Wang generalized gradient approximation (PW91). The HCTH functional provides a reasonably accurate description for carbon-diamond, although as the periodic table is descended the results become progressively less accurate. This reflects the predominance of low-atomic-number atoms in the fitting data used to determine the HCTH functional. HCTH electronic band gaps are generally larger than PW91 and LDA, but are still significantly below experiment.

Effect of exchange and correlation on bulk properties of MgO, NiO, and CoO

Bulk properties of the isostructural oxides MgO, NiO, and CoO have been calculated quantum chemically with periodic models and compared with experimental data from the literature. Ab initio Hartree-Fock, gradient-corrected density-functional methods, and hybrid approaches have been used for the calculation of the lattice constants, heats of atomization, and electronic structures. General trends of the effects of electron correlation and the treatment of exchange on the calculated properties are observed. None of the standard methods considered provided results in agreement with experimental data for all properties. A combination of Becke's three-parameter exchange functional and the Lee-Yang-Parr correlation functional (B3LYP) leads to a consistent description of the electronic and structural properties in comparative studies of the three compounds. Other methods are more accurate than B3LYP if certain properties or compounds are considered. The combination of the Hartree-Fock exchange functional with the Lee-Yang-Parr density-functional correlation is the best method for the open-shell transition-metal oxides NiO and CoO in terms of relative stability and geometry and the electronic structure of the valence band. The absolute values of calculated heats of atomization, however, are generally too small. The density-functional method based on the Perdew-Wang generalized gradient approximation (PWGGA) is preferable for the calculation of thermodynamic properties for all compounds but is less reliable in the prediction of structural and electronic properties. A hybrid approach based on the PWGGA method is proposed that improves the results for bulk geometries and electronic properties while maintaining the high quality of calculated energetic results.

Electronic structure of polyglycine as a function of proton position

Using a first-principles density-functional method, we have calculated the single-chain band structure for the α-helical protein polyglycine as a function of proton position along the N⋯H⋯O hydrogen bond. Calculations have been carried out using both the local-density approximation (LDA) and the Perdew–Wang generalized gradient approximation (GGA) for the exchange-correlation potential. We present results for the total energy, band structure, charge densities, and Mulliken charges as a function of proton position. The total energy gives an asymmetric double-well potential for simultaneous proton shift, with the global minimum (at a N–H distance of 0.95 Å) 2.02 eV (46.6 kcal/mol) below the top of the barrier with the GGA, and 1.83 eV (42.2 kcal/mol) below with the LDA.

Cohesive properties of the lanthanides: Effect of generalized gradient corrections and crystal structure

Two different approximations to the density functional, the local (spin) density approximation (LDA) in the Hedin-Lundqvist parametrization and the Perdew-Wang generalized gradient approximation (GGA), are compared using the lanthanide series plus barium as testing ground. Our total-energy calculations are parameter-free and all-electron, with a full-potential linear muffin-tin orbital basis set. The equilibrium volumes, bulk moduli, and generalized cohesive energies are calculated, assuming both the fcc and hcp crystal structures, and compared to experimental data. We find that GGA corrects most of the overbonding tendency of LDA for these elements. Our results also suggest that the standard model of the lanthanides, according to which the $4f$ shell can be viewed as chemically inert, is not fully appropriate for the early lanthanides (not including La), and that the trend in the bulk modulus is much less smooth than previously thought.

Diffusion Monte Carlo study of jellium surfaces: Electronic densities and pair correlation functions.

We present the results of fixed-node diffusion Monte Carlo calculations of jellium surfaces for metallic densities. We used a trial wave function of the Slater-Jastrow type, with the long-range part of the two-body term modified to account for the anisotropy of the system. The one-body term is optimized so that the electronic density from variational and diffusion Monte Carlo calculations agree with each other. The final densities are close to the densities obtained from density-functional calculations in the local-density approximation and the surface energies to the results obtained using the Langreth-Mehl and Perdew-Wang generalized gradient approximation at high densities ( r s 3.25) they agree with the results of the Fermi-hypernetted-chain calculations. The pair-correlation functions at regions near the surface are tabulated, showing the anisotropy of the exchange-correlation hole in regions of fast-varying densities. @S0163-1829~96!06848-8#