Generalized Gradient Approximation Results Research Articles

Hyperfine interactions for small systems including transition-metal elements using self-interaction corrected density-functional theory

The interactions between the electronic magnetic moment and the nuclear spin moment, i.e., magnetic hyperfine (HF) interactions, play an important role in understanding electronic properties of magnetic systems and in realizing platforms for quantum information science applications. We investigate the HF interactions for atomic systems and small molecules, including Ti or Mn, by using Fermi–Löwdin orbital (FLO) based self-interaction corrected (SIC) density-functional theory. We calculate the Fermi contact (FC) and spin-dipole terms for the systems within the local density approximation (LDA) in the FLO-SIC method and compare them with the corresponding values without SIC within the LDA and generalized-gradient approximation (GGA), as well as experimental data. For the moderately heavy atomic systems (atomic number Z ≤ 25), we find that the mean absolute error of the FLO-SIC FC term is about 27 MHz (percentage error is 6.4%), while that of the LDA and GGA results is almost double that. Therefore, in this case, the FLO-SIC results are in better agreement with the experimental data. For the non-transition-metal molecules, the FLO-SIC FC term has the mean absolute error of 68 MHz, which is comparable to both the LDA and GGA results without SIC. For the seven transition-metal-based molecules, the FLO-SIC mean absolute error is 59 MHz, whereas the corresponding LDA and GGA errors are 101 and 82 MHz, respectively. Therefore, for the transition-metal-based molecules, the FLO-SIC FC term agrees better with experiment than the LDA and GGA results. We observe that the FC term from the FLO-SIC calculation is not necessarily larger than that from the LDA or GGA for all the considered systems due to the core spin polarization, in contrast to the expectation that SIC would increase the spin density near atomic nuclei, leading to larger FC terms.

Exploring the structural, electronic, magnetic, and magneto-optical properties of double perovskites Ca2TMIrO6 (TM = Fe, Co) through first principles study

This study is aimed at exploring the electronic, magnetic, and magneto-optical properties of double perovskites Ca2FeIrO6 and Ca2CoIrO6 with monoclinic structure (space group P21/c) in order to examine their potential applications in spintronic and photovoltaic devices. The calculations were done using the full-potential linearized augmented plane wave within the density functional theory. For the electronic exchange-correlation function, we used the generalized gradient approximation (GGA) and GGA+U (Hubbard potential), and spin-orbit coupling (SOC). The study showed that Ca2FeIrO6 and Ca2CoIrO6 exhibit a monoclinic structure (space group P21/c). The structure relaxation shows an antiferromagnetic behavior in both systems with a magnetic moment of about 6.00 μB for Ca2FeIrO6 and 4.00 μB for Ca2CoIrO6 by using GGA+U approximation. The results of GGA and GGA+U predict the half-metallic behavior of Ca2FeIrO6 and Ca2CoIrO6. The magneto-optical polar Kerr effect (MOKE) was examined by studying the variation of Kerr and ellipticity rotation. The Kerr rotation angle is 1.3º at 4.82 eV and -1.21º at 4.3 eV, and the ellipticity angle is -1.21º at 4.3 eV for Ca2FeIrO6. In the case of Ca2CoIrO6, the Kerr rotation angle is -1.04º at 4.05 eV; the significant Kerr rotation in both compounds may suggest the application of these materials in optoelectronics bias. The named compounds have a potential application in the field of spintronics and its devices, such as in optoelectronics technologies.

Impurity properties in phosphorene: First-principles calculations and comparisons

Based on first-principles calculations within the generalized gradient approximation (GGA) and the hybrid functional (HSE06) frameworks, the properties of dozens of impurities in phosphorene have been systematically investigated and compared. The adsorbed Li, Na and K impurities have low formation energies and create defect states near the conduction band minimum of phosphorene, thus they are promising donors. The adsorbed Ga, In, Cu and Ag impurities also act as donors but have high formation energies, making them hard to be formed in equilibrium conditions. Group-II and group-VIII impurities introduce defect states in the energy gap far away from the band edges, while the adsorbed Zn, Cd and group-VI impurities produce no defect states in the gap region. According to the GGA results, the substitutional group-IV impurities create defect states near the valence band maximum, but in the HSE06 calculations these states shift deeply into the energy gap, rendering the impurities hard to be activated. Among the group-VI and group-VII impurities, only the adsorbed O, F and Cl have low formation energy. The formation energy of the same group impurities monotonously decreases or increases with the impurity-atom’s location in the periodic table of elements.

First-principles investigation of electronic and magnetic propertiess of double perovskites A2NRuO6 (A2 = Ba2, BaLa; N = V, Cr, Fe)

The electronic and magnetic properties of double perovskites A 2 NRuO 6 (A 2 = Ba 2 , BaLa; N = V, Cr, Fe) have been investigated using the first-principles density-functional theory (DFT) within the generalized gradient approximation (GGA) and GGA+U schemes. All compounds of (A 2 = Ba 2+ ) and doped (A 2 = Ba 2+ La 3+ ) crystallize in a cubic structure with (Fm-3m) space group. The GGA results of (A 2 = Ba 2+ ) predict half-metallic (HM), semiconductor and metallic character when (N = V, Cr, Fe), respectively which are completely stabilize at the HM nature within (A 2 = Ba 2+ La 3+ ) compounds. By including the exchange-correlation energy in GGA+U scheme, all compounds show a HM property, except for Ba 2 FeRuO 6 which appears as an insulating material. In addition, GGGA and GGA+U calculations of spin magnetic moments suggest a ferrimagnetic (FI) character for A 2 NRuO 6 (N = V and Cr), switch to a ferromagnetic (FM) nature when N = Fe. It is found that the two ions of N 3+ – Ru 5+ (A 2 = Ba 2 ) and N 3+ – Ru 4+ (A 2 = BaLa) are governed by the antiferromagnetic interactions N 3+ (3d)–O 2– (2p)–Ru 5+ ∕ Ru 4+ (4d) in high∕ low spin magnetic moments states.

Ab initio study of thermodynamic properties of bulk zinc-blende CdS: Comparing the LDA and GGA

In the present study, we have investigated the phonon and thermodynamic properties of bulk zinc-blende CdS by first-principle calculations within the density functional theory (DFT) and the density functional perturbation theory (DFPT) method using the quasi harmonic approximation (QHA). We calculated the phonon dispersion at several high symmetry directions, density of phonon state, temperature dependence feature of Helmholtz free energy (F), internal energy, bulk modulus, constant-volume specific heat, entropy, coefficient of the volume thermal expansion and Grüneisen parameter estimated with the local density approximation (LDA) and generalized gradient approximation (GGA) for the exchange-correlation potential and compared them with each other. For internal energy, Helmholtz free energy, constant volume heat capacity and phonon entropy the LDA and GGA results are very similar. But, the LDA gives lattice constants that are smaller than GGA while phonon frequencies, bulk modulus and cohesive energies are larger. On the other hand, the results obtained through the GGA approximation for the coefficient of the volume thermal expansion and Grüneisen parameter are larger than those obtained from LDA.

Electronic and Optical Properties of γ- and -Alumina by First Principle Calculations

The electronic and optical parameters of {\gamma}-Al2O3 and {\theta}-Al2O3 have been studied by using the first principle within the framework of density function theory (DFT). The computational approach is based on full-potential linearized augmented plane wave method (FP-LAPW) within the generalized gradient approximation (GGA), local density approximation (LDA), and modified Becke-Johnson potential (mBJ). The results show that these compounds have a direct gap ({\Gamma}-{\Gamma}) of about 5.375 eV and 4.716 eV for {\gamma}-Al2O3 and {\theta}-Al2O3, respectively. Several optical parameters of these materials are also investigated. The values of the real part of dielectric constant are found to be 3.259 and 3.694 for {\gamma}-Al2O3 and {\theta}-Al2O3, respectively, which are close to the experimental one (3.416). The refractive index is 1.806 and 1.922 for {\gamma}-Al2O3 and {\theta}-Al2O3 respectively, and shows a good agreement with the experimental result which is 1.86. GGA findings are consistent with the experimental results and are better than the other approximations. There are no salient differences between GGA and LDA results. The results advocate using this material as a transparent conducting layer in solar cell structure, which can be operated in a wide energy range.

Correlation corrected phonon vibration in YTiO3 and its effect on electronic properties

The infrared (IR) phonon vibration of the narrow t2g band system YTiO3 and its effect on electronic properties are studied using the generalized gradient approximation (GGA) and its combination with Coulomb correlation U. Coulomb U is found to improve the GGA results not only in electronic properties which has been researched frequently, but also in its lattice dynamics. The phonon frequency is increased effectively, especially in high-frequency region, which corrects the gap between experimental and theoretical GGA result. More importantly, IR-active modes are found to affect electronic properties significantly, and we find vibration-sensitive magnetic exchange interaction and strong electron-phonon coupling.

Improved half-metallic gap of zincblende half-metal superlattices with the Tran–Blaha modified Becke–Johnson density functional

Binary transition-metal pnictides and chalcogenides half-metallic ferromagnetic materials with zincblende structure, being compatible with current semiconductor technology, can be used to make high-performance spintronic devices. Here, we investigate electronic structures and magnetic properties of composite structure ((CrX)2/(YX)2 (X=As, Sb; Se, Te and Y=Ga; Zn) superlattices) of zincblende half-metallic ferromagnetism and semiconductor by using Tran and Blaha's modified Becke and Johnson (mBJ) exchange potential. Calculated results show that they all are half-metallic ferromagnets with both generalized gradient approximation (GGA) and mBJ, and the total magnetic moment per formula unit follows a Slater–Pauling-like “rule of 8”. The key half-metallic gaps by using mBJ are enhanced with respect to GGA results, which is because mBJ makes the occupied minority-spin p-bands move toward lower energy, but toward higher energy for empty minority-spin Cr-d bands. When the spin–orbit coupling (SOC) is included, the spin polarization deviates from 100%, and a most reduced polarization of 98.3% for (CrSb)2/(GaSb)2, which indicates that SOC has small effects, of the order of 1%, in the considered four kinds of superlattice.

Insights in the electronic structure and redox reaction energy in LiFePO4 battery material from an accurate Tran-Blaha modified Becke Johnson potential

The main goals of this paper are to investigate the accuracy of the Tran-Blaha modified Becke Johnson (TB-mBJ) potential to predict the electronic structure of lithium iron phosphate and the related redox reaction energy with the lithium deintercalation process. The computed electronic structures show that the TB-mBJ method is able to partially localize Fe-3d electrons in LiFePO4 and FePO4 which usually is a problem for the generalized gradient approximation (GGA) due to the self interaction error. The energy band gap is also improved by the TB-mBJ calculations in comparison with the GGA results. It turned out, however, that the redox reaction energy evaluated by the TB-mBJ technique is not in good agreement with the measured one. It is speculated that this disagreement in the computed redox energy and the experimental value is due to the lack of a formal expression to evaluate the exchange and correlation energy. Therefore, the TB-mBJ is an efficient method to improve the prediction of the electronic structures coming form the standard GGA functional in LiFePO4 and FePO4. However, it does not appear to have the same efficiency for evaluating the redox reaction energies for the investigated system.

A van der Waals density functional theory comparison of metal decorated graphene systems for hydrogen adsorption

Previous Density Functional Theory (DFT) studies on metal decorated graphene generally use local density approximation (LDA) or generalized gradient approximation (GGA) functionals which can cause inaccuracies in hydrogen binding energies as they neglect van der Waals (vdW) interactions and are difficult to compare due to their widely varying simulation parameters. We investigated the hydrogen binding ability of several metals with a consistent set of simulations using the GGA functional and incorporated vdW forces through the vdW-DF2 functional. Metal adatom anchoring on graphene and hydrogen adsorption ability for both single and double sided decoration were studied for eight metals (Al, Li, Na, Ca, Cu, Ni, Pd, and Pt). It was found that the vdW correction can have a significant impact on both metal and hydrogen binding energies. The vdW-DF2 functional led to stronger metal adatom and hydrogen binding for light metals in comparison to GGA results, while heavier transition metals displayed the opposite behaviour but still produced stronger hydrogen binding energies than light metals. Nickel was found to be the best balance between hydrogen binding ability for reversible storage and low weight. The effects on hydrogen binding energy and maximum achievable hydrogen gravimetric density were analyzed for Ni-graphene systems with varying metal coverage. Lower metal coverage was found to improve hydrogen binding but decrease hydrogen gravimetric density. The highest achieved Ni-graphene system gravimetric density was 6.12 wt. %.

Pressure-induced structure and properties of crystalline β-FOX-7 by LDA and GGA calculations

The structural, electronic, and absorption properties of crystalline β-FOX-7 were studied by density functional theory–generalized gradient approximation (GGA) and local density approximation (LDA) calculations. The results indicate that there are irregular changes occurred at 9, 17, 32, and 37 GPa for the GGA results and at 2, 7, 22, and 32 GPa for the LDA ones, in agreement with previous reports that the LDA generally underestimated, while the GGA overestimated the structure and properties of the crystals. The compression ratios of the lattice constants suggest that the structure of β-FOX-7 is much stiffer in a- and c-directions than in b-direction. The total decrease ratio of C–NO2 band lengths is larger than that of other bond lengths, indicating that the C–NO2 bond cleavage may trigger the initiation decomposition of β-FOX-7. An analysis of density of states suggests that the interactions between electrons are strengthened under the influence of pressure. The absorption spectra indicate that the bands in the fundamental absorption region have relatively high optical activity at high pressure.

The electronic structure and optical properties of XSi(X = Fe,Ru,Os): A first principles investigation within the modified Becke–Johnson exchange potential plus LDA

The electronic structure, optical reflectivity spectra and optical conductivity of semiconducting transition-metal silicides FeSi, RuSi and OsSi have been investigated by using first principles calculation within the recent developed modified Becke–Johnson exchange potential plus local-density approximation (mBJ+LDA). The electronic structures produced by mBJ+LDA, generalized gradient approximation (GGA) and LDA are rather similar although the band gap has been enlarged more or less by the mBJ+LDA compared to the GGA and LDA for the three compounds. The mBJ+LDA, GGA and LDA all have overestimated the band gap for FeSi and OsSi compared to the experiment. For RuSi, the theoretical gap values are basically close to the experimental values and the improvement of gap by mBJ+LDA is only 0.04eV in comparison with the GGA. The mBJ+LDA and GGA also produce similar results with respect to their optical properties including the reflectivity spectra and optical conductivity except that for the reflectivity spectra of FeSi, the GGA result is little better consistent with the experimental measurement than the mBJ+LDA result. The optical conductivity calculated by mBJ+LDA and GGA both exhibits the absorption edge, in well correspondence to the optical measurement.

Energy gaps and optical properties of ferromagnetic semiconductors [formula omitted] and [formula omitted]: Improved density-functional-theory study

We use a modified Becke–Johnson (mBJ) exchange plus local density approximation correlation functional within the density functional theory (DFT) [semi-local, orbital-independent, producing accurate semiconductor gaps, see Tran and Blaha, Phys. Rev. Lett., 102 (2009) 226401] to investigate the electronic structures of CdCr2S4 and CdCr2Se4 as two ferromagnetic semiconductors. Our results show that mBJ makes the empty Cr d-eg character bands higher by about 1.1eV for CdCr2S4 and 0.7eV for CdCr2Se4 for the majority-spin channel with respect to the generalized gradient approximation (GGA). For the minority-spin channel, mBJ raises the Cr d character bands by about 0.8eV for CdCr2S4 and CdCr2Se4. The mBJ gap is enhanced by about 104% for CdCr2S4 and 170% for CdCr2Se4 compared to GGA results, and is consistent with experiment in both of the cases. Our mBJ dielectric functions and the zero frequency refractive index n(0) for CdCr2S4 and CdCr2Se4 are also consistent with experimental results available. The improved DFT results can be understood in terms of mBJ-enhanced spin exchange splitting and the reduced band width of Cr d character bands.

Ab initio molecular dynamics simulation of pressure-induced phase transformation in BeO

Ab initio molecular dynamics (MD) method has been used to study high pressure-induced phase transformation in BeO based on the local density approximation (LDA) and the generalized gradient approximation (GGA). Both methods show that the wurtzite (WZ) and zinc blende (ZB) BeO transforms to the rocksalt (RS) structure smoothly at high pressure. The transition pressures obtained from the LDA method are about 40 GPa larger than the GGA result for both WZ → RS and ZB → RS phase transformations, and the phase transformation mechanisms revealed by the LDA and GGA methods are different. For WZ → RS phase transformations both mechanisms obtained from the LDA and GGA methods are not comparable to the previous ab initio MD simulations of WZ BeO at 700 GPa based on the GGA method. It is suggested that the phase transformation mechanisms of BeO revealed by the ab initio MD simulations are affected remarkably by the exchange–correlation functional employed and the way of applying pressure.

First-principles investigation of magnetism and electronic structures of substitutional3dtransition-metal impurities in bcc Fe

The magnetic and electronic structures of $3d$ impurity atoms from Sc to Zn in ferromagnetic body-centered cubic iron are investigated using the all-electron full-potential linearized augmented plane-wave method based on the generalized gradient approximation (GGA). We found that in general, the GGA results are closer to the experimental values than those of the local spin density approximation. The calculated formation enthalpy data indicate the importance of a systematic study on the ternary Fe-C-$X$ systems rather than the binary Fe-$X$ systems, in steel design. The lattice parameters are optimized and the conditions for spin polarization at the impurity sites are discussed in terms of the local Stoner model. Our calculations, which are consistent with previous work, imply that the local spin-polarizations at Sc, Ti, V, Cu, and Zn are induced by the host Fe atoms. The early transition-metal atoms couple antiferromagnetically, while the late transition-metal atoms couple ferromagnetically, to the host Fe atoms. The calculated total magnetization ($M$) of bcc Fe is reduced by impurity elements from Sc to Cr as a result of the antiferromagnetic interaction, with the opposite effect for solutes which couple ferromagnetically. The changes in $M$ are attributed to nearest neighbor interactions, mostly between the impurity and host atoms. The atom averaged magnetic moment is shown to follow generally the well-known Slater-Pauling curve, but our results do not follow the linearity of the Slater-Pauling curve. We attribute this discrepancy to the weak ferromagnetic nature of bcc Fe. The calculated Fermi contact hyperfine fields follow the trend of the local magnetic moments. The effect of spin-orbit coupling is found not to be significant although it comes into prominence at locations far from the impurity sites.

AunPtnclusters adsorbed on graphene studied by first-principles calculations

In this work, we present the adsorption of ${\text{Au}}_{n}{\text{Pt}}_{n}$ clusters on graphene surface by using the density-functional theory within localized density approximation (LDA), and generalized gradient approximation (GGA). We calculate the adsorption energy, geometries, band structure, charge differences, and magnetization up to $n=3$. We observe that graphene can be metallic or semiconducting depending on number of Au and Pt atoms in the cluster and the charge transfer between cluster and the graphene. LDA and GGA results are given and compared for all cases. We find that ${\text{Au}}_{3}{\text{Pt}}_{3}$ and AuPt graphene can have half metallic properties within GGA.

Structural properties and phase transformations under pressure of XTe compounds (X = Be, Mg, and Ca): The role of the exchange–correlation potential

We have performed first principles total energy calculations to investigate the structural properties and possible phase transitions under pressure of IIA–VI compounds: BeTe, MgTe and CaTe. We have considered the following possible structures: rock-salt, nickel arsenide, cesium chloride, zinc-blende, and in some cases wurtzite. Calculations are done using the periodic density functional theory. We employ the full potential linearized augmented plane wave method as implemented in the wien2k code. The exchange and correlation potential energies are treated according to the generalized gradient approximation (GGA) using the Perdew, Burke, Ernzerhof (PBE) parameterization, and the local density approximation (LDA). Our results show that the GGA calculations correctly predict the ground state structure of all three binary compounds: zinc-blende for BeTe, wurtzite/zinc-blende for MgTe, and rock-salt for CaTe. Under pressure, BeTe and MgTe transform to the nickel arsenide structure, while CaTe transforms from rock-salt to cesium chloride. Slightly different results are found using the LDA approximation. We discuss the role of the ionicity in the difference between the LDA and GGA results.

First-principles study of the pentacene/Cu(1 1 1) interface: Adsorption states and vacuum level shifts

We have studied the interaction of pentacene with a Cu(1 1 1) surface using density functional theory (DFT) within a generalized gradient approximation (GGA) and the van der Waals density functional [vdW-DF, M. Dion, H. Rydberg, E. Schröder, D.C. Langreth, B.I. Lundqvist, Phys. Rev. Lett. 92 (2004) 246401]. The adsorption energy is accurately predicted by vdW-DF, while the equilibrium distances between pentacene and the metal substrate ( Z C) are overestimated by both GGA and vdW-DF. The work function changes depend significantly on Z C. The experimental work function change can be successfully reproduced by GGA if the experimentally reported adsorption geometry is used, whereas the magnitude of the work function change is underestimated if calculated adsorption geometries are applied. We examined the IDIS model [H. Vázquez, R. Qszwaldowski, P. Pou, J. Ortega, R. Pérez, F. Flores, A. Kahn, Europhys. Lett. 65 (2004) 802] to compare it with the GGA results. The interface dipoles estimated by the IDIS model fairly agree with the GGA results, provided that the adsorption distance is large. On the other hand, they tend to deviate from the GGA results as the adsorption distance becomes smaller, where back donation from the metal surface to the adsorbate occurs. Our analysis reveals that at experimentally reported metal–organic distance, back donation is significant enough to induce polarization of pentacene molecules perpendicular to the surface, which leads to a reduction of the work function. Thus, at the experimentally reported metal–organic distance, the work function change estimated by a simple IDIS model deviates from that calculated by self-consistent GGA calculations. We also found that at the experimentally reported metal–organic distance, the transferred electrons create weak chemical bonds between pentacene and the Cu(1 1 1) surface, illustrating the reactive nature of pentacene.

High-pressure behavior of crystalline FOX-7 by density functional theory calculations

1,1-Diamino-2,2-dinitroethylene (C2H4N4O4, FOX-7) is a newly found energetic materials with high performance and low sensitivity. We performed density functional theory (DFT) calculations within local density approximation (LDA) as well as generalized gradient approximation (GGA) to simulate the structural and electronic properties of FOX-7 crystal under high-pressure up to 4GPa. Due to the insufficient treatment of weakly intermolecular interaction, LDA underestimates the lattice parameters and volume, while GGA overestimates them. It is interesting to find that a mixing of LDA and GGA results of lattice properties can lead to fairly good agreement with experiments. Examination of pressure-induced changes of molecular geometry shows that the C–N bonds are most sensitive under compression. The change of band gap as function of external pressure was also discussed.

First-Principles Langevin Molecular Dynamics Studies of Metallic and Semiconductor Clusters: GGA versus LDA Results

The combination of Langevin molecular dynamics for simulated annealing with realistic quantum-mechanical interactions obtained from first principles supercell calculations within the pseudo potential plane-wave method are applied to examine the structural and electronic properties of the following systems i) pure and mixed GenTem (n, m = 0 - 2), Ge3, Te3 and Ge3Te3 semiconductor clusters, and ii) bimetallic PbNan (n ≤ 7) clusters. Results using the local density approximation (LDA) and the generalized gradient approximation (GGA) for the exchange correlation functional are compared with available experimental data. It is found that the GGA leads to larger bond lengths and smaller binding energies than the LDA, but both approximations lead to the same group symmetry for the ground state of these clusters. Two isomeric configurations for PbNa6, having C3v and Oh symmetries, respectively, are found. The dipole moments of semiconductor clusters are compared with those estimated from recent experiments at room temperature. The dipole moments from GGA calculations are, on average, in better agreement with experiments than those calculated with the LDA.