GGA Calculations Research Articles

Microcalorimetric, infrared spectroscopic and DFT studies of CO adsorption on Rh and Rh–Te catalysts

Microcalorimetric measurements of the adsorption of CO on a Rh/0.9Te/SiO 2 catalyst were performed at 203, 253, and 303 K. The initial heats of CO adsorption were approximately 87 kJ/mol, and this value is much weaker than the value of 160 kJ/mol measured on a Rh/SiO 2 catalyst. Infrared spectra were collected for CO adsorbed on Rh/SiO 2, Rh/0.7Te/SiO 2, and Rh/0.9Te/SiO 2 catalysts. The primary IR bands were located at frequencies near 2030 cm −1 for the Rh/Te/SiO 2 catalysts, compared to the higher frequency of 2080 cm −1 for CO on Rh/SiO 2. In addition, the intensity of the IR band for CO on the Rh/Te/SiO 2 catalysts increased irreversibly when the samples were warmed from 203 to 303 K. Periodic self-consistent DFT–GGA calculations were carried out to study the adsorption of CO on slabs of Rh(111), RhTe(0001), and RhTe(11 2 ̄ 0). These calculations show that CO adsorbs strongly on Rh(111), Rh-terminated RhTe(0001), and RhTe(11 2 ̄ 0) surfaces, with heats higher than 160 kJ/mol. In contrast, weaker adsorption was calculated for adsorption of CO on Te-terminated RhTe(0001) surfaces, with a heat near 70 kJ/mol. Thus, the Te-terminated RhTe(0001) surface appears to be the best model surface for Rh/0.9Te/SiO 2 catalyst, based on the combined results of microcalorimetric measurements and DFT calculations. Adsorption of CO on the hcp site of the Te-terminated RhTe(0001) surface leads to a reconstruction in which the Rh atom bonded to CO is pulled from its position in the second layer to a location above the surface. A lower limit for the activation energy barrier of this CO-induced surface reconstruction is 44 kJ/mol (which corresponds to the barrier of the more exothermic O-induced surface reconstruction). This CO-induced surface reconstruction may be the origin for the experimentally observed increase in the intensity of the IR band for adsorbed CO when Rh/Te/SiO 2 samples are warmed from 203 to 303 K in the presence of CO.

Ionization of HCl and HF in ice: a periodic DFT study

We present the results of periodic DFT–GGA calculations on the HCl and HF ionization inside a cubic ice structure. The model considers the substitution of a water molecule by an HX molecule. The potential energy curves are drawn at fixed H–X distances. HCl dissociation inside the ice structure is a barrierless process, while HF presents a typical dissociation potential curve. The selected computational methodology is able to reproduce the experimental observations.

Chemistry of Sulfur Oxides on Transition Metals I: Configurations, Energetics, Orbital Analyses, and Surface Coverage Effects of SO2 on Pt(111)

An extensive search for stable chemisorbed SO2 configurations on the Pt(111) surface has been performed using first-principles DFT−GGA calculations. The most energetically stable configurations, η2-Sb,Oa and η3-Sa,Oa,Oa at fcc sites, where η2 and η3 mean that the number of atoms of the adsorbate coordinated to surface atoms are two and three, respectively, and the subscripts a and b stand for the atoms on atop sites and bridge sites, respectively, are consistent with experimental observations. It is found that strong sulfur−metal bonds are essential in stabilizing the molecular SO2 binding to the Pt(111) surface. The lateral dipole−dipole interactions among chemisorbed SO2 moieties are shown to be responsible for the strong coverage effect of the SO2 binding energy to the surface. These strong lateral interactions do not greatly affect the molecular structures or relative binding energy differences among different binding configurations. The projected density of states and the induced density of states are studied in detail to explain binding effects.

Methanol Decomposition on Cu(111): A DFT Study

Self-consistent periodic DFT–GGA calculations are used to investigate the methanol decomposition pathway on both equilibrium and stretched Cu(111) surfaces. The thermochemistry and kinetics of the decomposition via sequential hydrogen abstraction are both found to be highly unfavorable. The second step in this pathway, the abstraction of hydrogen from the methoxy intermediate, has large thermochemical (∼0.6 eV) and kinetic (∼1.4 eV) barriers. Our thermochemical results indicate that methanol, formaldehyde, and carbon monoxide are weakly bound to the surface and will likely desorb before undergoing any reaction under UHV conditions. In contrast, methoxy, formyl, and atomic hydrogen are much more strongly bound. Introduction of a 4% lateral strain to the Cu(111) lattice decreased the transition state energy of the methoxy hydrogen abstraction step, suggesting that strain might facilitate the kinetics of this reaction.

Full-potential LMTO study on the electronic structure of heavy-fermion compound LiV2O4

The electronic structure of heavy fermion compound LiV2O4 has been calculated using a self-consistent full-potential LMTO method. The results show that the conduction bands in this compound are formed from V 3 d states with a bandwidth of 2.5 eV. The symmetric characteristics of conduction bands are of t2g in principle. The energy gap between conduction bands and fully occupied oxygen 2p bands is 1.9 eV. The band dispersions near Fermi energy display complicated structures. Furthermore, the N( EF) and ycal are 11.1 (states/eV/f.u.) and 26.7 mJ/mol.K2 determined numerically by LDA calculation, respectively. It is insufficient to clarify the origin of local moment in LiV2O4 from plain LDA calculations. In addition to the above LDA calculation, we also found a LSDA solution of LiV2O4 that is lower in total energy than that of LDA calculation. Similarly, LSDA + GGA calculation yields almost the identical result as that in LSDA. We conclude that the mechanism responsible for heavy fermion properties in LiV2O4 might be somewhat different from the plain Kondo mechanism in conventional 4f and 5f heavy fermion compounds and perhaps the quantum transition might play an adequate role in heavy-ferrnion behaviors in LiV2O4.

First principles calculation of thermal expansion coefficient: Part 1. Cubic metals

Based on the Debye–Grüneisen approximation, we conducted coefficient of thermal expansion (CTE,α) calculations for nine pure cubic metals (Pd, Cu, Ag, Rh, V, Nb, Rb, K and Al) using density functional theory. Employing Pd, Cu, and Ag as benchmark, we compared the numerical performance of CTE for four density functional theory (DFT) methods, namely LDA, LSDA, GGA and GGS. For these three metals, we found that the gradient-corrected methods (GGA and GGS) yields much larger CTE compared to the local-density methods (LDA and LSDA). By comparing with experimental CTE, we found that the GGA approach yields the best estimate for this property when a ‘cutoff radius’ of 10% is used. For the nine pure cubic metals investigated here, the mean-absolute-deviation of the GGA CTE is found to be less than 3% from the experimental value (except for Al), which represents a major improvement over previous LDA-based calculation. To further explore the applicability of the developed CTE calculation protocol, we extend the GGA calculation to eight compounds (AlNi, AgMg, AuCd, GaNi, Pd3Sn, AlNi3, AuCu3 and Al3U) with cubic structures. Good agreement with experiment was obtained, indicating the feasibility of applying the same technique on ordered alloys.

Structural and electronic structure properties of FeSi: the drivingforce behind the stability of the B20 phase

We present the results of a first-principles pseudopotential plane-wave studyfor the structural properties of the ε-FeSi (B20), NaCl (B1) andCsCl (B2) structures of FeSi. The calculations were performed using the localdensity and the generalized gradient approximations (LDA and GGA), for theexchange-correlation potential. The electronic structures of the B1and B2 phases have been similarly investigated. These calculations haveenabled us to identify the driving force behind the crystallization of FeSi inthe B20 phase. Both the B1 and B2 phases are found to be semimetallic, withthe Fermi energy lying in a pseudo-band-gap. The B20 structure is predicted tobecome unstable with respect to the B2 phase at a moderate pressure, of 13.5and 10.9 GPa according to the GGA and LDA calculations, respectively.

The mechanism for the 3×3 distortion of Sn/Ge(111)

We show that two distinct 3×3 ground states — one non-magnetic, metallic and distorted; the other magnetic, semi-metallic (or insulating) and undistorted — compete in α-phase adsorbates on semiconductor (111) surfaces. In Sn/Ge(111), local (spin) density approximation (LSDA) and GGA calculations indicate, in agreement with experiment, that the distorted metallic ground state prevails. The reason for the stability of this state is analysed, and traced to a sort of bond density wave, specifically a modulation of the antibonding state filling between the adatom and a GeGe bond directly underneath.

LDA and GGA calculations of alkali metal adsorption at the (001) surface of MgO

The adsorption geometry, binding energy and electronic structure of alkali metal overlayers on the MgO (001) surface have been studied by means of density functional theory, using Gaussian-type orbitals to expand the wave functions and electronic charge density. A two-dimensionally periodic slab of MgO with alkali metal adsorbed at one surface was used to model the semi-infinite system. Li, Na, and K were considered at both half- and quarter-monolayer coverage. Results were compared for the local density approximation and for two different forms of the generalized gradient approximation. In all cases Li was found to interact with the surface approximately twice as strongly as Na and three times as strongly as K. The epitaxial binding energies were, however, always less than or close to the bulk cohesive energies of the respective alkali metals, suggesting an instability of the adsorbed film toward the formation of two- or three-dimensional islands, in agreement with experiment. Spin polarized and unpolarized calculations were compared to detect metal–insulator transitions in the alkali overlayer. Only Li adsorbed at 1:4 coverage was found to have lower energy in a spin polarized (hence nonmetallic) state.

Fermi surface-analysis of YBa 2Cu 3O 7 by full-potential LAPW calculations

We report on the shape of the Fermi surface of YBa 2Cu 3O 7 based on both LDA and GGA calculations using optimized geometry data. In addition, we analyze the electronic states at the Fermi level in terms of atomic contributions thus focusing on hybridization effects.

Electronic band structure and lattice distortion in perovskite transition-metal oxides

The electronic structure of LaMO 3 (M  Ti, V, …, Cu) are reviewed with particular attention to the effect of the lattice distortion. The FLAPW and LMTO methods are employed in the band structure calculations in the LSDA, GGA and LDA + U schemes. The experimental lattice parameters are used in these calculations. The change of the bond angle of MOM gives rise to the change of the width of the d band. For LaVO 3 and LaMnO 3 with cooperative Jahn-Teller distortions, the LDA and/or GGA calculations produce orbital orders, which determine the types of spin orders. The LaTiO 3 system has no Jahn-Teller distortion, where the spin-orbit coupling might play an important role.

Density-functional calculation of the electronic structure and equilibrium geometry of iron pyrite (FeS2)

The electronic structure and equilibrium geometry of the pyrite form of ${\mathrm{FeS}}_{2}$ was studied using self-consistent density-functional theory comparing the local-density-approximation (LDA) and the generalized-gradient-approximation (GGA) forms of the exchange-correlation functional. The calculated contour map of the electron-deformation density is consistent with analysis of high-resolution x-ray-diffraction data, showing excess charge in nonbonding d states on the Fe sites. At the experimentally measured geometry, the cohesive energy is calculated to be 16.7 eV/${\mathrm{FeS}}_{2}$ and 9.4 eV/${\mathrm{FeS}}_{2}$ for the LDA and GGA forms, respectively; the GGA result comparing well with the experimental value of 10.7 eV/${\mathrm{FeS}}_{2}$. Optimizing the cohesive energy as a function of geometry, both the LDA and GGA calculations find the optimal structure to be expanded relative to the experimental geometry with the lattice constant expanded by 1% and the S-S bond length expanded by 6%, corresponding to a 0.1-eV increase in the cohesive energy for each ${\mathrm{FeS}}_{2}$ unit. Similar results were obtained for ${\mathrm{RuS}}_{2}$.