Gradient-corrected Density Functional Theory Research Articles

Covalency in the f Element−Chalcogen Bond. Computational Studies of M[N(EPR2)2]3 (M = La, Ce, Pr, Pm, Eu, U, Np, Pu, Am, Cm; E = O, S, Se, Te; R = H, iPr, Ph)

The geometric and electronic structures of the title complexes have been studied using scalar relativistic, gradient-corrected density functional theory. Extension of our previous work on six-coordinate M[N(EPH 2) 2] 3 (M = La, Ce, U, Pu; E = O, S, Se, Te), models for the experimentally characterized M[N(EP (i)Pr 2) 2] 3, yields converged geometries for all of the other 4f and 5f metals studied and for all four group 16 elements. By contrast, converged geometries for nine-coordinate M[N(EPPh 2) 2] 3 are obtained only for E = S and Se. Comparison of the electronic structures of six- and nine-coordinate M[N(EPH 2) 2] 3 suggests that coordination of the N atoms produces only minor changes in the metal-chalcogen interactions. Six-coordinate Eu[N(EPH 2) 2] 3 and Am[N(EPH 2) 2] 3 with the heavier group 16 donors display geometric and electronic properties rather different from those of the other members of the 4f and 5f series, in particular, longer than expected Eu-E and Am-E bond lengths, smaller reductions in charge difference between M and E down group 16, and larger f populations. The latter are interpreted not as evidence of f-based metal-ligand covalency but rather as being indicative of ionic metal centers closer to M (II) than M (III). The Cm complexes are found to be very ionic, with very metal-localized f orbitals and Cm (III) centers. The implications of the results for the separation of the minor actinides from nuclear wastes are discussed, as is the validity of using La (III)/U (III) comparisons as models for minor actinide/Eu systems.

Water Clusters Confined in Nonpolar Cavities by Ab Initio Calculations

Encapsulation of (H2O)n clusters (n = 1−22) in fullerene cages of different diameters (0.73−1.41 nm) has been investigated using gradient-corrected density functional theory. A linear relationship between cavity volume and maximum number of the encapsulated water molecules has been obtained. The interaction between water molecules and the fullerene wall was identified as physisorption with an adsorption energy of about 1.1 kcal/mol per molecule. The equilibrium configurations of small confined water clusters (n < 12) roughly resemble those of gas-phase clusters, whereas larger water clusters tend to adopt cage-like configurations when they are encapsulated in fullerene cages of sufficiently large diameter (i.e., >1.4 nm). The dipole moments of water clusters in the confined phase are smaller than those in the gas phase due to the screening effect of the outer fullerene cage. These results might shed some light on the behavior of water clusters confined in the nonpolar cavities of biological interests.

First-principles study of TMNan (TM = Cr, Mn, Fe,Co, Ni; n = 4–7) clusters

Geometry, electronic structure and magnetic properties ofTMNan (TM = Cr–Ni;n = 4–7) clusters are studied within a gradient-corrected density functional theory (DFT)framework. Two complementary approaches, the first adapted to all-electroncalculations on free clusters and the second based on the plane-wave projectoraugmented wave (PAW) method within a supercell approach, are used. Except forNiNan, the clusters in this series are found to retain the atomic moments of the TM atoms,and the magnetic moment presented an odd–even oscillation with respect to thenumber of Na atoms. The origin of these odd–even oscillations is explained fromthe nature of the chemical bonding in these clusters. Differences and similaritiesbetween the chemical bonding and the magnetic properties of these clusters and theTMNan (TM = Sc, Vand Ti; n = 4–6) clusters on the one hand and TM-doped Au and Ag clusters on the other hand arediscussed.

Ab Initio Molecular Dynamics Study of the Solvated OHCl− Complex: Implications for the Atmospheric Oxidation of Chloride Anion to Molecular Chlorine

We have studied the OHCl(-) complex in a six-water cluster and in bulk liquid water by means of Born−Oppenheimer molecular dynamics based on generalized gradient-corrected BLYP density functional theory. Self-interaction-corrected results, which predict a hydrogen-bonded OH···Cl(-) complex, are compared to the uncorrected results, which predict a hemibonded (HO-Cl)(-). A second-order Møller−Plesset potential energy landscape of the gas-phase complex in its ground-state was computed to determine which of the two configurations represents the true nature of the complex. Because no evidence of a local minimum was found in the vicinity of the geometry corresponding to (HO-Cl)(-), we conclude that the self-interaction-corrected results are more accurate and, therefore, that the complex is held together by a hydrogen-bond-like interaction in both an asymmetric solvation environment, as represented by the cluster, and a symmetric solvation environment, as represented by the bulk system. We postulate that the mechanism that governs the atmospheric oxidation of Cl(-)(aq) to Cl(2)(g) on the surface of marine aerosols is initiated by the formation of a H-bonded OH···Cl(-) complex. Furthermore, because no evidence of charge transfer from Cl(-) to OH was found, in either the liquid or the cluster environment, we propose that the second step of the oxidation of Cl(-) is the reaction of the complex with a second Cl(-), resulting in the formation of the species Cl(2)(-) and OH(-). Cl(2)(g) could then be formed via an electron-transfer reaction with an impinging OH molecule.

Cage and tube structures of medium-sized zinc oxide clusters (ZnO)n (n=24, 28, 36, and 48)

Following our recent work which revealed that the lowest-energy structures of (ZnO)n (n=9-18) follow cage and tube structural growth patterns with stacks of small subunits of (ZnO)2 and (ZnO)3 [Wang et al., J. Phys. Chem. C 111, 4956 (2007)], we have extended the search for the most stable structures to some larger clusters, i.e., (ZnO)n (n=24, 28, 36, and 48) by using gradient-corrected density-functional theory (DFT). A number of starting configurations belonging to different structural motifs were generated from handmade constructions with chemical intuition and then optimized via DFT calculations. Within the size range studied, cage and tube structures were found to be the most preferred structural motifs for the (ZnO)n clusters.

Density functional theory studies of inorganic metallocene multidecker Vn(P6)n+1 (n=1–4) sandwich clusters

Motivated by the synthesis of the first entirely inorganic metallocene sandwich ion [eta(5)-Ti-(P(5))(2)](2-) [E. Urnezius et al. Science 295, 832 (2002)], we have designed a new inorganic metallocene sandwich [eta(6)-V-(P(6))(2)] and multidecker sandwich clusters up to V(4)(P(6))(5) by employing an all electron gradient-corrected density functional theory. The binding energies of the V(n)(P(6))(n+1) complexes increase rapidly from half sandwich to the smallest full sandwich and become gradually afterwards. The highest occupied and lowest unoccupied molecular orbital gap and the vertical ionization energy decrease with increasing cluster size. The V(n)(P(6))(n+1) clusters are nonferromagnetic and prefer the lowest available spin states. The smallest sandwich cluster, V(P(6))(2), has the high stability and might serve as a building block for one-dimensional inorganic polymers with high stabilities.

Structural and electronic properties of Bin (n = 2-14) clusters from density-functional calculations

The structural and electronic properties of Bi n (n = 2-14) clusters have been systematically studied using gradient-corrected density-functional theory. For each cluster size, a number of structural isomers were constructed and optimized to search for the lowest-energy structure. The competition of several structural patterns such as cages, superclusters, and layered structures leads to the alternating appearance of these configurations as global minima. Although the tendency of Bi to form puckered-layer structures is already well-known, the electronic states of Bi n clusters are still far from that of the bulk. As well, a remarkable even-odd atom number oscillation is observed in the structural and electronic properties of the clusters, implying that the stability of Bi n clusters is mainly dominated by the electron shell effect rather than by geometrical packing. The theoretically calculated values for electron affinities agree well with available experimental data.

Consistent set of band parameters for the group-III nitrides AlN, GaN, and InN

We have derived consistent sets of band parameters (band gaps, crystal field splittings, band-gap deformation potentials, effective masses, and Luttinger and ${E}_{P}$ parameters) for AlN, GaN, and InN in the zinc-blende and wurtzite phases employing many-body perturbation theory in the ${G}_{0}{W}_{0}$ approximation. The ${G}_{0}{W}_{0}$ method has been combined with density-functional theory (DFT) calculations in the exact-exchange optimized effective potential approach to overcome the limitations of local-density or gradient-corrected DFT functionals. The band structures in the vicinity of the $\ensuremath{\Gamma}$ point have been used to directly parametrize a $4\ifmmode\times\else\texttimes\fi{}4$ $\mathbf{k}\ensuremath{\cdot}\mathbf{p}$ Hamiltonian to capture nonparabolicities in the conduction bands and the more complex valence-band structure of the wurtzite phases. We demonstrate that the band parameters derived in this fashion are in very good agreement with the available experimental data and provide reliable predictions for all parameters, which have not been determined experimentally so far.

Stone−Wales Defects in Single-Walled Boron Nitride Nanotubes: Formation Energies, Electronic Structures, and Reactivity

The geometries, formation energies, electronic properties, and reactivities of Stone−Wales (SW) defects in single-walled (8,0) boron nitride nanotubes (BNNTs) were investigated by means of gradient-corrected density functional theory (DFT) computations. SW defects deform BNNTs severely and result in local curvature changes at defect sites. The energies of defect formation increase with increasing tube diameters and are orientation dependent. Depending on the SW defect orientations, additions to central 7-7 ring fusions can be either more or less favorable than to defect-free sites. The reaction energies of model H2 addition are mostly endothermic for defective as well as pristine BNNTs, but reactions at the most favorable sites near SW defect (homoelement N−N bonds followed by the B−B bond sites) are exothermic. This and the fact that the band structures of BNNTs are only slightly changed by SW and vacancy defects as well as by chemical additions at low modification ratios endow BNNTs with great application potential.

Stabilizing the silicon fullerene Si24 by La and Gd encapsulation

Using gradient-corrected spinpolarized density-functional theory (DFT) for electronic structure calculations，we study the fullerene of silicon Si24 with Th and D2d symmetries in the neutral state stabilized by lanthanum and gadolinium encapsulation. The electronic and magnetic properties of La@Si24 and Gd@Si24 are calculated using spin unrestricted DFT. The higher spin magnetic moment is achieved for Gd@Si24 and the magnetic moment of La@Si24 is completely quenched. These results could possibly lead to new phases and derivatives of silicon.

Structural, Electronic, and Magnetic Properties of Con(benzene)m Complexes

The structural, electronic, and magnetic properties of cobalt-benzene complexes (Co(n)Bz(m), n, m = 1-4, m = n, n + 1) have been explored within the framework of an all electron gradient-corrected density functional theory. Sandwich conformations are energetically preferred for the smallest series of n, m = 1-2, rice-ball structures are for larger sizes with n > or = 3, and both motifs coexist for Co(2)Bz(3). The rice-ball clusters of (3, 3) and (4, 4) are more stable than (3, 4) having a relative large binding energy and HOMO-LUMO gap whereas smaller sandwich clusters have highly kinetic stability at (n, n + 1). The computed ionization potentials and magnetic moments of Co(n)Bz(m) are in good agreement with the measured values overall; the present results suggest that the measured moments are averages reflecting mixtures of a few nearly isoenergetic isomers having different spin states. The magnetism of the complexes mainly comes from Co atoms with a Bz molecule only possessing very small moments. Ferromagnetic ordering is energetically preferred for smaller complexes with n = 1-3 whereas antiferromagnetic ordering is favored for (4, 4). The relatively smaller moments of Con clusters in a Bz matrix indicate that Bz molecules play an attenuation role to the magnetism of the complexes.

Structure, energetics, and bonding of amorphous Au–Si alloys

First principles periodic calculations based on gradient-corrected density functional theory have been performed to examine the structure, energetics, and bonding of amorphous Au-Si alloys with varying Au:Si composition ratios. Our results predict that the Au-Si alloy forms the most stable structure when the Si content is around 40-50 at. %, with an energy gain of about 0.15 eV/atom. In addition, the volume change per atom in the alloy exhibits a distinctive nonlinear trend, with the minimum value around 60 at. % Si. The occurrence of the minimum in the Au-Si mixing energy and volume is attributed to strong hybridization of the Au 5d-Si 3p states. We also present variations in the radial distribution function and atomic coordination number as a function of Au:Si composition ratio, with discussion of the nature of local packing and chemical bonding in the Au-Si alloy system.

Improved dipole moments by combining short-range gradient-corrected density-functional theory with long-range wave-function methods

Previously proposed schemes of coupling short-range density-functional-based with long-range wave-function-based methods are tested for the notoriously difficult case of correctly describing the charge distribution in compounds containing late transition elements. We show that for the dipole moments of the group-11 transition metal hydrides and halides the recently developed Coulomb-attenuated Becke three-parameter Lee-Yang-Parr hybrid functional already leads to a substantial improvement compared to other density functionals. Further improvement is achieved by combining a gradient-corrected short-range functional of the Perdew-Burke-Ernzerhof type with coupled-cluster theory. The results clearly demonstrate that mixing of long-range ab initio Hartree-Fock and post-Hartree-Fock methods helps to remove deficiencies of current density functionals.

A DFT + U description of oxygen vacancies at the TiO 2 rutile (1 1 0) surface

Experimental observations indicate that removing bridging oxygen atoms from the TiO 2 rutile (1 1 0) surface produces a localised state approximately 0.7 eV below the conduction band. The corresponding excess electron density is thought to localise on the pair of Ti atoms neighbouring the vacancy; formally giving two Ti 3+ sites. We consider the electronic structure and geometry of the oxygen deficient TiO 2 rutile (1 1 0) surface using both gradient-corrected density functional theory (GGA DFT) and DFT corrected for on-site Coulomb interactions (GGA + U) to allow a direct comparison of the two methods. We show that GGA fails to predict the experimentally observed electronic structure, in agreement with previous uncorrected DFT calculations on this system. Introducing the + U term encourages localisation of the excess electronic charge, with the qualitative distribution depending on the value of U. For low values of U (⩽4.0 eV) the charge localises in the sub-surface layers occupied in the GGA solution at arbitrary Ti sites, whereas higher values of U (⩾4.2 eV) predict strong localisation with the excess electronic charge mainly on the two Ti atoms neighbouring the vacancy. The precise charge distribution for these larger U values is found to differ from that predicted by previous hybrid-DFT calculations.

Multimetallocenes. A Theoretical Study

Quantum chemical calculations using gradient-corrected density functional theory at the BP86 level in conjunction with TZ2P basis sets have been carried out for the multimetallocenes CpMnCp, where M = Be, Mg, Ca, and Zn with n = 2−5. The equilibrium geometries and energetics with respect to loss of one metal atom are theoretically predicted. The nature of the metal−ligand interactions between the Mn2+ and (Cp-)2 moieties was investigated with energy decomposition analysis (EDA). The calculations predict that the CpMnCp species with n > 2 are thermodynamically unstable with respect to loss of one metal atom except for the beryllium compounds. The beryllocenes exhibit unusual stabilities in the gas phase for the whole series CpBenCp up to n = 5. The calculations suggest that the energy for loss of one metal atom from CpBe2Cp is significantly higher than from CpZn2Cp. The energy for the metal extrusion reaction of CpBe3Cp is much less endothermic than for CpBe2Cp but it is still more endothermic than the rea...

On the Sequential Hydrogen Dissociative Chemisorption on Small Platinum Clusters: A Density Functional Theory Study

We present a simple cluster model to understand the dissociative chemisorption of molecular hydrogen and the desorption of atomic hydrogen on platinum small clusters using the gradient-corrected density functional theory. Successive H2 decomposition and sequential H desorption on the selected Ptn (n = 2−5, 7−9) clusters were systematically studied, and the H2 dissociative chemisorption energies and the H desorption energies at the full H saturation were identified. The reaction processes are driven by charge transfer from Pt atoms to H atoms assisted by strong orbital overlaps between Pt 5d orbitals and H 1s orbital, which leads to electron delocalization in large clusters of metal hydrides. It was found that the number of H atoms chemisorbed on the small Pt clusters increases almost linearly with the size of the selected Pt cluster.

First-principles calculation of vibrational frequencies for monoclinic tribismuth borate BiB3O6

Monoclinic BiB3O6 is an excellent nonlinear optical material with many advantages compared to other borate crystals. As one part of understanding the properties of BiB3O6, the harmonic frequencies of BiB3O6 are calculated by applying the numerical-difference technique based on first-principles gradient-corrected hybrid B3PW density functional theory electronic structure calculations within the Gaussian-orbital-based CO-LCAO scheme. The complete 13A and 14B vibrational modes are assigned, graphically visualized and classified according to the Bi—O and B—O motions. Comparisons with previous experimental reports are discussed in detail.

Selectivity of Direct Ethanol Fuel Cell Dictated by a Unique Partial Oxidation Channel

Self-consistent periodic slab calculations based on gradient-corrected density functional theory (DFT-GGA) were conducted to investigate the potential energy diagram for ethanol oxidation over Pt(111). Ethanol oxidation on Pt is found to be dominated by an one-step concerted dehydrogenation pathway to produce acetaldehyde, which is about 600 times faster than the traditionally regarded stepwise pathways. The same mechanism is transferable to methanol oxidation but with a minor contribution. The results can help to clarify the long-standing puzzles on the selectivity of direct alcohol fuel cell over Pt.

Gold‐Coated Transition‐Metal Anion [Mn13@Au20]‐ with Ultrahigh Magnetic Moment.

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A Quantum Chemical Study of the Haptotropic Rearrangements of Cr(CO)3 on Naphthalene and Phenanthrene Systems

First-principles gradient-corrected density functional theory electronic structure calculations of the haptotropic rearrangement of a Cr(CO)(3) unit on naphthalene and phenanthrene derivatives are reported. Coupled-cluster calibration studies of Cr(CO)(3) complexes with benzene and naphthalene derivatives confirm the accuracy of the applied Becke exchange and Perdew correlation functionals. Characteristic points on the energy hypersurface (reactants, products, intermediates, and transition states) were located for various substituents on the aromatic skeleton. It is argued that a -OH/-O(-) substituent may provide a means to steer the haptotropic shift depending on the pH value, i.e., to construct a molecular switch. In addition, the mechanism of the [3 + 2 + 1] benzannulation of chromium pentacarbonyl naphthylcarbene complexes with alkynes was investigated, and the preference of the angular benzannulation leading to phenanthrene complexes of Cr(CO)(3) over the linear benzannulation leading to corresponding anthracene complexes is explained.