Density Functional Theory Electronic Structure Research Articles

Theoretical predictions of trends in spectroscopic properties of homonuclear dimers and volatility of the 7p elements

Fully relativistic density functional theory electronic structure calculations were performed for homonuclear dimers of the 7p elements, 113-118 and their 6p homologs, Tl through Rn. All the dimers of the heaviest elements, with the exception of (118)(2), were found to be weaker bound than their lighter homologs. The difference in the dissociation energy (D(e)) between the 6p and 7p homologs was shown to decrease from group 15 to group 17, with a reversal of the trend in group 18. A remarkable feature is a shift of the maximum in D(e)(M(2)) from group 15 in the third through sixth rows to group 16 in the seventh row. Strong relativistic effects on the 7p atomic orbitals, particularly, their large spin-orbit splitting, were shown to be responsible for these trends. Using the calculated D(e)(M(2)), the sublimation enthalpies, DeltaH(sub), of macroamounts, or formation enthalpies of gaseous atoms, DeltaH(f)(g), of the heaviest elements were estimated using a linear correlation between these quantities in the chemical groups. The newly estimated values are in good agreement with those obtained via a linear extrapolation from the lighter homologs in the groups.

Very large scale wavefunction orthogonalization in Density Functional Theory electronic structure calculations

Enforcing the orthogonality of approximate wavefunctions becomes one of the dominant computational kernels in planewave based Density Functional Theory electronic structure calculations that involve thousands of atoms. In this context, algorithms that enjoy both excellent scalability and single processor performance properties are much needed. In this paper we present block versions of the Gram–Schmidt method and we show that they are excellent candidates for our purposes. We compare the new approach with the state of the art practice in planewave based calculations and find that it has much to offer, especially when applied on massively parallel supercomputers such as the IBM Blue Gene/P Supercomputer. The new method achieves excellent sustained performance that surpasses 73 TFLOPS (67% of peak) on 8 Blue Gene/P 1 racks (32 768 compute cores), while it enables more than a two fold decrease in run time when compared with the best competing methodology.

An aromatic imine group enhances the EL efficiency and carrier transport properties of highly efficient blue emitter for OLEDs

This study not only describes the synthesis and characterization of a novel indenofluorene derivative, 6,6,12,12-tetraethyl-2,8-bis-[1,1′;3′,1′]terphenyl-4′-yl-6,12-dihydro-indeno[1,2-b]fluorine (TP-EIF), for use as an emitting material in blue organic light emitting diodes (OLEDs), but also the comparison with 6,6,12,12-tetraethyl-2,8-bis-[1,1′;3′,1′]terphenyl-4′-yl-6,12-dihydro-diindeno[1,2-b;1′,2′-e]pyrazine (TP-EPY). UV-visible and PL spectra showed that the TP-EPY absorption and emission bands were red-shifted with narrow full widths at half maxima (FWHMs) compared to TP-EIF, both in solution and as a film. Density functional theory electronic structure calculations suggested that the spectral differences arose from the electron withdrawing properties of the imine group in the indenopyrazine, which increased conjugation in the core group and decreased conjugation in the side group at the LUMO level. The lower energy of the LUMO in TP-EPY resulted in increased active electron injection in electron-only devices and cyclic voltammetry (CV) measurements. The hole mobilities, which were measured by time of flight (TOF) methods, were 1.8 × 10−3 and 1.1 × 10−4 cm2 V−1 s for TP-EPY and TP-EIF, respectively, which demonstrated that TP-EPY displayed excellent hole transport abilities. The electron mobility was also high in TP-EPY (8.3 × 10−5 cm2 V−1 s). TP-EPY was, therefore, shown to be bipolar, unlike TP-EIF. The electroluminescence (EL) performance of TP-EPY in OLED devices was higher than that of TP-EIF: the luminance efficiencies were 1.72 and 0.76 cd A−1, and the power efficiencies were 0.65 and 0.27 lm W−1, for TP-EPY and TP-EIF, respectively. In particular, the FWHMs of the EL spectral bands were 54 nm for TP-EPY and 73 nm for TP-EIF, which demonstrated enhanced emission spectral purity. The external quantum efficiency of TP-EPY was 5.1%, which was more than twice the value for TP-EIF (2.19%).

Prediction of Iron-Isotope Fractionation Between Hematite (α-Fe2O3) and Ferric and Ferrous Iron in Aqueous Solution from Density Functional Theory

Density functional theory electronic structure calculations are used to compute equilibrium constants for iron-isotope exchange among Fe2+(aq), Fe3+(aq), and hematite (alpha-Fe2O3). The hematite is represented in both bulk and surface environments. The iron-isotope fractionation between Fe2+(aq) and Fe3+(aq), determined using a range of exchange-correlation functionals and basis sets, is in good agreement with experimental measurements. The calculated reduced partition function ratio for bulk hematite is very close to previous estimates based on Mossbauer and inelastic nuclear resonance X-ray spectroscopy. However, the calculated fractionation between hematite bulk and the aqueous species Fe3+(aq) and Fe2+(aq) differs from experimental measurements carried out at the aqueous-hematite interface. We find a heavy iron enrichment trend in the order Fe2+(aq) < hematite bulk approximately hematite surface < Fe3+(aq). In contrast to experimental studies, we find a significant positive fractionation (heavy enrichment) for Fe(3+)(aq) relative to hematite, regardless of whether the hematite is represented by a bulk or a surface model. Our calculations indicate that it is unlikely that the aqueous interfacial structure of hematite is a simple termination of the bulk structure.

Calcium-hydroxyl group complex for potential hydrogen storage media: A density functional theory study

Using density functional theory electronic structure calculations, we investigate the calcium-hydroxyl group complex for potential applications to the hydrogen storage at near ambient temperature and pressure. The Ca atom is bound to the hydroxyl group with a binding energy comparable to the cohesive energy of bulk Ca, and we find that each Ca atom binds up to seven ${\text{H}}_{2}$ molecules in the molecular form. Binding of an unexpectedly large number of ${\text{H}}_{2}$ molecules is attributed to the fact that $d$ orbitals of Ca positive ions are downshifted and partially occupied, thereby validating the empirical 18-electron rule as in the transition metal atoms. The binding energy turns out to be $\ensuremath{\sim}0.1\text{ }\text{eV}/{\text{H}}_{2}$, somewhat smaller than the requirement $(\ensuremath{\ge}0.2\text{ }\text{eV}/{\text{H}}_{2})$ of the room-temperature application. We also show that an important bonding mechanism of ${\text{H}}_{2}$ molecules on Ca is the polarization, namely, the electric dipole moment of ${\text{H}}_{2}$ induced by the partially ionized Ca. Based on this result, the possibility of the organic material functionalized with hydroxyl groups for a hydrogen storage medium is discussed.

Density functional characterization of the electronic structure and optical properties of Cr-doped SrTiO3

First-principles density functional theory electronic structure calculations were carried out for Cr-doped SrTiO3 to evaluate the effect of Cr-doping on the band gap states and the photocatalytic activity of SrTiO3. The defect formation energy is also investigated to determine the energy required for Cr-doping. Our results indicate that substituting Cr for Sr requires smaller formation energy than substituting Cr for Ti in the Cr-doped SrTiO3 structures and the Cr atoms energetically prefer to partially take up some Sr sites, simultaneously partially some Ti sites. Some Cr 3d gap states appear near the bottom of the conduction band in the Cr-doped SrTiO3 structures, which results in the decreased electron transition energy and thus the visible light absorption observed in the experiment. The correlations between the doped Cr atoms at different sites and the environmental O atoms may be responsible for the differences in the visible light photocatalytic activities for the doped SrTiO3. Further research demonstrates that different dopant concentrations in Cr-doped SrTiO3 do not bring significant changes in the electronic characteristics but may change a little the electron transition energy and weaken photocatalytic activity under visible light.

Experimental and Theoretical Characterization of the Magnetic Properties of CuF2(H2O)2(pyz) (pyz = pyrazine): A Two-Dimensional Quantum Magnet Arising from Supersuperexchange Interactions through Hydrogen Bonded Paths

The structural, electronic, and magnetic properties of the new linear chain coordination polymer CuF2(H2O)2(pyz) (pyz = pyrazine) were determined by single crystal X-ray diffraction at various temperatures, SQUID magnetometry, pulsed-field magnetization, ESR, muon-spin relaxation (μSR), and electronic structure calculations. Each Cu2+ ion of CuF2(H2O)2(pyz) is located at a distorted CuF2O2N2 octahedron with axial elongation along the Cu−N bonds. These octahedra are tethered together by strong F···H−O hydrogen bonds to yield two-dimensional (2D) square nets in the bc-plane that are linked along the a-direction by pyrazine linkages. Measurements of the g-factor by ESR along with first principles density functional theory electronic structure calculations show that the magnetic orbital of the Cu2+ ion lies in the CuF2O2 plane thus forming a 2D antiferromagnetic square lattice. A broad maximum observed in χ(T) at 10 K indicates a modest spin exchange interaction through the Cu−F···H−O−Cu supersuperexchange pa...

Enhanced Ferromagnetism and Tunable Saturation Magnetization of Mn/C-Codoped GaN Nanostructures Synthesized by Carbothermal Nitridation

Mn/C-codoped GaN nanostructures were synthesized by carbothermal nitridation with active charcoal as the carbon source. Nanostructures such as zigzag nanowires and nanoscrews were observed by varying the reaction time and the C/Ga molar ratio of the starting material used for the synthesis. The structures and morphologies of the as-grown samples were characterized by X-ray diffraction, scanning electron microscopy, and high-resolution transmission electron microscopy measurements. The doping of both Mn and C in the GaN matrix was confirmed by X-ray photoelectron spectroscopy measurements, and the ferromagnetic properties of Mn/C-codoped GaN samples were confirmed by room-temperature magnetization measurements. The saturation magnetization of Mn/C-codoped GaN increases steadily with increasing C/Ga molar ratio of the starting material at a rate of approximately 0.023 emu/g per C/Ga molar ratio, and the ferromagnetism of Mn/C-codoped GaN can be stronger than that of Mn-doped GaN by a factor of approximately 40. A plausible growth mechanism was proposed, and the role of carbon codoping in tuning the morphology and ferromagnetic property was discussed. Our work suggests that carbon doping in the GaN matrix favors the N sites over the Ga sites, Mn/C-codoping in the GaN matrix is energetically favorable, and the C-codoping strongly enhances the preference of the FM coupling to the AFM coupling between the two doped Mn sites. These suggestions were probed on the basis of first-principles density functional theory electronic structure calculations for a number of model doped structures constructed with a 32-atom 2 x 2 x 2 supercell.

Electronic structure and optical properties of poly[3‐(4‐octylphenoxy)thiophene]: Experimental and theoretical studies

AbstractThis article reports the synthesis and characterization of a new polythiophene derivative phenoxy‐substituted, the poly[3‐(4‐octylphenoxy)thiophene] (POPOT). The oxidative polymerization was found to yield low molecular weight material, whereas a modified Grignard metathesis (GRIM) yielded polymers of high molecular weights. One‐ and two‐dimensional NMR indicated the latter to be highly regioregular. POPOTs exhibited higher thermal stabilities than equivalent alkoxy‐substituted polythiophenes and exhibited red shifts in the absorption spectra with respect to equivalent. The absorption spectra showed a red shifted λmax at 540 nm in tetrahydrofuran solutions and 580 nm in spin‐coated films, with respect to poly(3‐alkylthiophene)s. A further red shift of 40 nm in going from solution (540 nm) to solid states (580 nm) is correlated with results from density functional theory electronic structure calculations. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 7505–7516, 2008

ChemInform Abstract: Can Undoped Calcium Tetraborides Exist? An Answer from the Comparison of Its Density Functional Theory Electronic Structure with that of Rare‐Earth Metal Tetraboride.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

First-Principles-Based Calculations of Vibrational Normal Modes in Polyatomic Materials with Translational Symmetry: Application to PETN Molecular Crystal

Numerical studies of vibrational energy transport and associated (non)linear infrared and Raman response in polyatomic materials require knowledge of the multidimensional vibrational potential-energy surface and the ability to perform normal-mode analysis on that potential. The presence of translational symmetry, as in crystals, leads to the observed dispersion of the unit cell normal modes and has to be accounted for in calculations of energy transfer rates and other spectroscopic quantities. Here we report on the implementation of a computational approach that combines the generalized supercell method and density functional theory electronic structure calculations to investigate the vibrational structure in translationally symmetric materials containing relatively large numbers of atoms in the unit cell (58 atoms in the present study). The method is applied to calculate the phonon and vibron dispersion relations and the vibrational density of states in pentaerythritol tetranitrate (PETN) molecular crystal which is an important energetic material. The results set the stage for future investigations of vibrational energy transport and associated nonlinear spectroscopic signatures in this class of materials.

Density Functional Characterization of the Band Edges, the Band Gap States, and the Preferred Doping Sites of Halogen-Doped TiO2

First-principles density functional theory (DFT) electronic structure calculations were carried out for model halogen-doped anatase TiO2 structures to evaluate the effect of halogen doping on the band edges and the photocatalytic activity of TiO2. The model structures of X-doped TiO2 were constructed by using the 48-atom 2 × 2 × 1 supercell of anatase TiO2 with one O or one Ti atom replaced with X (=F, Cl, Br, I). The unit cell parameters and the atom positions of the resulting X-doped TiO2 with X at an O site (X@O) and that with X at a Ti site (X@Ti) were optimized by performing first-principles DFT calculations. On the basis of the optimized structures of X-doped TiO2 with X@O and X@Ti, the defect formation energies and the plots of the density of states were calculated to analyze the band edges, the band gap states, and the preferred doping sites. Our work shows that the doping becomes more difficult in the order F < Cl < Br < I for X-doped TiO2 with X@O, while the doping becomes less difficult in the ...

Origin of the Reactivity on the Nonterminated (100), (110), and (111) Diamond Surfaces: An Electronic Structure DFT Study

The reactivity of nonterminated diamond low-index surfaces has been evaluated using density functional theory (DFT). The intrinsic electronic structures of the topmost carbon atoms of diamond (100)−1 × 1, (100)−2 × 1, (110)−1 × 1, (111)−1 × 1, and (111)−2 × 1 surfaces have been calculated and analyzed using highly accurate numerical basis set first-principle techniques. The following reactivity indicators have been utilized: Fukui functions, electrostatic potential, and Kohn−Sham orbitals (highest occupied and lowest unoccupied). Their spatial representations have been mapped onto a charge density isosurface whereby plausible reactive sites were identified and related. Specifically, the change in chemically reactivity induced by a 1 × 1 to 2 × 1 surface reconstruction has been discussed. In addition, density of states and the deformation density of the topmost carbon atoms have been included. Most often the sites of electrophilically susceptible areas correspond to the mapping of f− function, electrostati...

Axial Ligand Effects on the Geometric and Electronic Structures of Nonheme Oxoiron(IV) Complexes

A series of complexes [Fe(IV)(O)(TMC)(X)](+) (where X = OH(-), CF3CO2(-), N3(-), NCS(-), NCO(-), and CN(-)) were obtained by treatment of the well-characterized nonheme oxoiron(IV) complex [Fe(IV)(O)(TMC)(NCMe)](2+) (TMC = tetramethylcyclam) with the appropriate NR4X salts. Because of the topology of the TMC macrocycle, the [Fe(IV)(O)(TMC)(X)](+) series represents an extensive collection of S = 1 oxoiron(IV) complexes that only differ with respect to the ligand trans to the oxo unit. Electronic absorption, Fe K-edge X-ray absorption, resonance Raman, and Mossbauer data collected for these complexes conclusively demonstrate that the characteristic spectroscopic features of the S = 1 Fe(IV)=O unit, namely, (i) the near-IR absorption properties, (ii) X-ray absorption pre-edge intensities, and (iii) quadrupole splitting parameters, are strongly dependent on the identity of the trans ligand. However, on the basis of extended X-ray absorption fine structure data, most [Fe(IV)(O)(TMC)(X)](+) species have Fe=O bond lengths similar to that of [Fe(IV)(O)(TMC)(NCMe)](2+) (1.66 +/- 0.02 A). The mechanisms by which the trans ligands perturb the Fe(IV)=O unit were probed using density functional theory (DFT) computations, yielding geometric and electronic structures in good agreement with our experimental data. These calculations revealed that the trans ligands modulate the energies of the Fe=O sigma- and pi-antibonding molecular orbitals, causing the observed spectroscopic changes. Time-dependent DFT methods were used to aid in the assignment of the intense near-UV absorption bands found for the oxoiron(IV) complexes with trans N3(-), NCS(-), and NCO(-) ligands as X(-)-to-Fe(IV)=O charge-transfer transitions, thereby rationalizing the resonance enhancement of the nu(Fe=O) mode upon excitation of these chromophores.

The electronic structure and energetics of V+-benzene half-sandwiches of different multiplicities: Comparative multireference and single-reference theoretical study

Multireference [complete active space self-consistent field (CASSCF) and multiconfigurational quasidegenerate perturbation theory (MCQDPT)] and single-reference ab initio (Moller-Plesset second order perturbation theory (MP2) and coupled clusters with singles, doubles and noniterative triples [CCSD(T)]) and density functional theory (PBE and B3LYP) electronic structure calculations of V(C(6)H(6))(+) half-sandwich in the states of different multiplicities are described and compared. Detailed analyses of the geometries and electronic structures of the all found states are given; adiabatic and diabatic dissociation energies are estimated. The lowest electronic state of V(C(6)H(6))(+) half-sandwich was found to be the quintet (5)B(2) state with a slightly deformed upside-down-boat-shaped benzene ring and d(4) configuration of V atom, followed by a triplet (3)A(2) state lying about 4 kcal/mol above. The lowest singlet state (1)A(1)(d(4)) lies much ( approximately 28 kcal/mol) higher. MCQDPT calculated adiabatic dissociation energy (53.6 kcal/mol) for the lowest (5)B(2)(d(4)) state agrees well with the current 56.4 (54.4) kcal/mol experimental estimate, giving a preference to the lower one. Compared to MCQDPT, B3LYP hybrid exchange-correlation functional provides the best results, while CCSD(T) performs usually worse. Gradient-corrected PBE calculations tend to systematically overestimate metal-benzene binding in the row quintet<triplet<singlet.

Characterization of the Magnetic and Structural Properties of Copper Carbodiimide, CuNCN, by Neutron Diffraction and First-Principles Evaluations of Its Spin Exchange Interactions

The crystal structure of copper carbodiimide, CuNCN, was determined from neutron diffraction data at room temperature and at 4 K, and the electrical resistivity, specific heat, and magnetic susceptibility measurements were carried out. The spin exchange interactions of CuNCN were evaluated by performing first-principles density functional theory electronic structure calculations. CuNCN is a semiconductor containing Jahn−Teller distorted CuN6 octahedra around the divalent copper ions, and the material shows a very small and almost temperature-independent magnetic susceptibility. Our electronic structure calculations evidence that the spin exchange interactions of CuNCN are dominated by two antiferromagnetic spin exchange paths leading to a triangular lattice antiferromagnet within the ab plane. Because the coupling between the layers (along the c axis) is small, CuNCN may be regarded a two-dimensional S = 1/2 frustrated triangular Heisenberg quantum antiferromagnet.

Can Undoped Calcium Tetraborides Exist? An Answer from the Comparison of Its Density Functional Theory Electronic Structure with that of Rare-Earth Metal Tetraboride

Periodic density functional theory calculations are used to discuss the existence of metal tetraborides MB4 with divalent metals. Tetraborides which contain metal atoms inserted in a three-dimensional boron network made of B6 octahedra and B2 dumbbells exhibit a pseudo energy gap for a count of 60 valence electrons per M4(B6)2(B2)2 formula unit. Such a count satisfies the stability electron requirement for B6(2-) (20 electrons) octahedra and B2(2-) (8 electrons) units and allows the filling of two supplementary low-lying bands deriving from the valence metallic d atomic orbitals. This favored electron count is not reached for CaB4 which is then formally deficient by one electron per metal atom. This indicates that CaB4 is unlikely to exist without n-doping.

Linking density functional and density-matrix theory: Picosecond electron relaxation at the Si(100) surface

We describe an approach that links the density-matrix theory for electron transport and relaxation with the density-functional theory for electronic structure. Our analysis of the electron dynamics at Si(100) reveals an unanticipated phonon bottleneck between bulklike and surface states. The fastest relaxation process observed in recent two-photon photoemission experiments is in good agreement with the calculated phonon-mediated intrasurface-band scattering.

Synthesis, X-ray crystallographic characterization, and electronic structure studies of a di-azide iron(III) complex: implications for the azide adducts of iron(III) superoxide dismutase.

We have synthesized and characterized, using X-ray crystallographic, spectroscopic, and computational techniques, a six-coordinate diazide Fe (3+) complex, LFe(N 3) 2 (where L is the tetradentate ligand 7-diisopropyl-1,4,7-triazacyclononane-1-acetic acid), that serves as a model of the azide adducts of Fe (3+) superoxide dismutase (Fe (3+)SOD). While previous spectroscopic studies revealed that two distinct azide-bound Fe (3+)SOD species can be obtained at cryogenic temperatures depending on protein and azide concentrations, the number of azide ligands coordinated to the Fe (3+) ion in each species has been the subject of some controversy. In the case of LFe(N 3) 2, the electronic absorption and magnetic circular dichroism spectra are dominated by two broad features centered at 21 500 cm (-1) (approximately 4000 M (-1) cm (-1)) and approximately 30 300 cm (-1) (approximately 7400 M (-1) cm (-1)) attributed to N3 (-) --> Fe (3+) charge transfer (CT) transitions. A normal coordinate analysis of resonance Raman (RR) data obtained for LFe(N 3) 2 indicates that the vibrational features at 363 and 403 cm (-1) correspond to the Fe-N 3 stretching modes (nu Fe-N3) associated with the two different azide ligands and yields Fe-N 3 force constants of 1.170 and 1.275 mdyne/A, respectively. RR excitation profile data obtained with laser excitation between 16,000 and 22,000 cm (-1) reveal that the nu Fe-N3 modes at 363 and 403 cm (-1) are preferentially enhanced upon excitation in resonance with the N 3 (-) --> Fe (3+) CT transitions at lower and higher energies, respectively. Consistent with this result, density functional theory electronic structure calculations predict a larger stabilization of the molecular orbitals of the more strongly bound azide due to increased sigma-symmetry orbital overlap with the Fe 3d orbitals, thus yielding higher N 3 (-) --> Fe (3+) CT transition energies. Comparison of our data obtained for LFe(N 3) 2 with those reported previously for the two azide adducts of Fe (3+)SOD provides compelling evidence that a single azide is coordinated to the Fe (3+) center in each protein species.

Structure and Magnetic Properties of Oxychalcogenides A2F2Fe2OQ2(A = Sr, Ba; Q = S, Se) with Fe2O Square Planar Layers Representing an Antiferromagnetic Checkerboard Spin Lattice

The oxychalcogenides A2F2Fe2OQ2 (A = Sr, Ba; Q = S, Se), which contain Fe2O square planar layers of the anti-CuO2 type, were predicted using a modular assembly of layered secondary building units and subsequently synthesized. The physical properties of these compounds were characterized using magnetic susceptibility, electrical resistivity, specific heat, (57)Fe Mossbauer, and powder neutron diffraction measurements and also by estimating their exchange interactions on the basis of first-principles density functional theory electronic structure calculations. These compounds are magnetic semiconductors that undergo a long-range antiferromagnetic ordering below 83.6-106.2 K, and their magnetic properties are well-described by a two-dimensional Ising model. The dominant antiferromagnetic spin exchange interaction between S = 2 Fe(2+) ions occurs through corner-sharing Fe-O-Fe bridges. Moreover, the calculated spin exchange interactions show that the A2F2Fe2OQ2 (A = Sr, Ba; Q = S, Se) compounds represent a rare example of a frustrated antiferromagnetic checkerboard lattice.