Showing papers on "Charge density published in 1995"

Density-functional theory and strong interactions: Orbital ordering in Mott-Hubbard insulators

Abstract: Evidence is presented that within the density-functional theory orbital polarization has to be treated on an equal footing with spin polarization and charge density for strongly interacting electron systems. Using a basis-set independent generalization of the LDA+U functional, we show that electronic orbital ordering is a necessary condition to obtain the correct crystal structure and parameters of the exchange interaction for the Mott-Hubbard insulator ${\mathrm{KCuF}}_{3}$.

Characterization of c-h-o hydrogen-bonds on the basis of the charge-density

Abstract: It is shown that the total charge density is a valid source to confirm hydrogen bonding without invoking a reference charge density. A set of criteria are proposed based on the theory of “atoms in molecules” to establish hydrogen bonding, even for multiple interactions involving C-H-O hydrogen bonds. These criteria are applied to several van der Waals complexes. Finally a bifurcated intramolecular C-H-O hydrogen bond is predicted in the anti-AIDS drug AZT, which may highlight a crucial feature of the biological activity of a whole class of anti-AIDS drugs. Almost all the methods of physical chemistry, spectroscopy, and diffraction can be used to recognize and study hydrogen bonding.] Each technique focuses on specific properties in order to detect and characterize this phenomenon in its own way. This work is concerned with the manifestation of hydrogen bonding in the charge density obtained from ab initio calculations. Whereas crystallographers have concluded upon hydrogen bonding via purely geometrical criteria, recent deformation density2 studies allow one to observe hydrogen bonding beyond mere ge~metry.~ However, it is not necessary to subtract an arbitrary (promolecular) charge density from the total density to reveal hydrogen bonding, not even in the interpretation of X-ray experiment^.^ Boyd and Choi have shown in two important contribution^^^^ that the theory of “atoms in molecules’’ (AIM)7,8 can be used to characterize hydrogen bonding solely from the (total) charge density for a large set of acceptor molecules, involving HF and HC1 as donors. In a next stage Carroll and Bader performed a more extended analysis on a large set of BASE-HF comple~es.~ This theory has not only provided new insights in conventional intermolecular hydrogenI0.’ ] bonding but has also been successful in intramolecularI33l4 and x-type hydrogen bonds.I5 Drawing from earlier ob~ervations~~~~ ~.’~~~~ and the present work, we formulate eight concerted effects occurring in the charge density which are indicative of hydrogen bonding. All of these effects can be viewed as necessary criteria to conclude that hydrogen bonding is present. By observation one of these conditions has proven to be sufficient as well. This case study on C-H-O interactions shows that this less common type of hydrogen bonding obeys all of the proposed criteria. Moreover, the multiple interactions appearing in the present five examples do not impair the consistency of the global phenomenon of hydrogen bonding as it expresses itself in the charge density. In spite of an early affirmative infrared review,I6 the old controversy on whether C-H-O hydrogen bonds really exist continued for another decade,” but now the dust has settled’* (for an entertaining account of this controversy, see ref 19). The importance of these bonds has been recognized in crystal engineering’9,20 since C-H-O contacts have a determining influence on packing motifs.21

Electro-osmosis on inhomogeneously charged surfaces.

Abstract: The electro-osmotic flow generated by an electric field ${\stackrel{\ensuremath{\rightarrow}}{E}}_{\mathrm{ext}}$ in a fluid bounded by surfaces bearing a charge varying in space is considered. Focusing on a slab geometry, I characterize the formation of steady convective rolls and describe their morphology as a function of the slab thickness. These rolls can be used to generate net currents and forces, even for zero average surface charge density. Moreover, the current (or the force) can be perpendicular to the applied field, opening the way to a variety of microscopic electromechanical devices.

Density dependent hadron field theory

Abstract: A fully covariant approach to a density dependent hadron field theory is presented. The relation between in-medium NN interactions and field-theoretical meson-nucleon vertices is discussed. The medium dependence of nuclear interactions is described by a functional dependence of the meson-nucleon vertices on the baryon field operators. As a consequence, the Euler-Lagrange equations lead to baryon rearrangement self-energies which are not obtained when only a parametric dependence of the vertices on the density is assumed. It is shown that the approach is energy-momentum conserving and thermodynamically consistent. Solutions of the field equations are studied in the mean-field approximation. Descriptions of the medium dependence in terms of the baryon scalar and vector density are investigated. Applications to infinite nuclear matter and finite nuclei are discussed. Density dependent coupling constants obtained from Dirac-Brueckner calculations with the Bonn NN potentials are used. Results from Hartree calculations for energy spectra, binding energies, and charge density distributions of $^{16}\mathrm{O}$, $^{40,48}\mathrm{Ca}$, and $^{208}\mathrm{Pb}$ are presented. Comparisons to data strongly support the importance of rearrangement in a relativistic density dependent field theory. Most striking is the simultaneous improvement of charge radii, charge densities, and binding energies. The results indicate the appearance of a new ``Coester line'' in the nuclear matter equation of state.

Electrostatic decoupling of periodic images of plane‐wave‐expanded densities and derived atomic point charges

Abstract: The density of a molecule expanded in plane waves is decoupled from its periodic images using a fit to atom‐centered Gaussians, which reproduces the long‐range electrostatic potential of the original density. The interaction energy between the cluster and its periodic images is calculated by an Ewald summation. The method has been applied to self‐consistent ab initio molecular dynamics calculations of charged and polar molecules. An atomic point charge model of the charge density is obtained, which can be used for classical molecular dynamics models and to couple classical and quantum mechanical simulations.

Particle charging in low‐pressure plasmas

Abstract: Particles embedded in a plasma acquire a net charge as a result of collisions with electrons and ions. Due to the stochastic nature of encounters between particle and charged species, the instantaneous charge fluctuates. The static properties of the charge fluctuations are quantified for particles surrounded by an undisturbed plasma in orbital motion limit. For particles that satisfy the condition e2/4πe0RkTe≪1 the charge distribution is a Gaussian function whose average and variance is related to the ion and electron currents toward the particle. For a Maxwellian plasma, in particular, analytical solutions are developed for the average charge and the variance as a function of the parameters of the plasma ne/ni, Te/Ti, and Me/Mi. Finally, the methodology is extended to non‐Maxwellian plasmas using the Druyvesteyn as an example.

Molecular dynamics simulation of an excess charge in water using mobile Gaussian orbitals

Abstract: For mixed quantum‐classical molecular dynamics simulations of solvated excess charges a novel and efficient method to expand the solute electronic wave function in a distributed Gaussian basis with a shell structure is presented. The aggregate of Gaussian orbitals is capable of mimicking the shape fluctuation of the excess charge distribution and its diffusion through the solvent. This approach also offers an easy pathway to treat the solvent electronic polarization in an explicit and self‐consistent fashion. As applications, the results of adiabatic molecular dynamics simulations for the hydrated electron and the aqueous chloride are reported. For e−/H2O the computed ground state absorption spectrum is discussed. Adiabatic relaxation as well as nonadiabatic transition rates are evaluated—the latter by means of an original Golden Rule formula—and compared to experimental results. In the case of Cl−/H2O the charge transfer to solvent spectra are analyzed. The ability of the mobile basis set method to descr...

Evaluation of pore structure and electrical properties of nanofiltration membranes

Abstract: Structural parameters and electrical properties of nanofiltration membranes (Desal-5, NF-40, NTR7450 and G-20) were determined with permeation experiments of aqueous solutions of neutral solutes (alcohols and saccharides having different molecular weights) and sodium chloride. The pore radii of these nanofiltration membranes were estimated to range from 0.4 to 0.8 nm based on the steric-hindrance pore model. Neither the constant surface charge density nor the constant surface electrical potential were suitable for interpreting NaCl concentration dependency of reflection coefficients. The fixed charge density of these membranes was evaluated on the basis of the Teorell-Meyer-Sievers model; a simple empirical equation was proposed to represent the dependency of the fixed charge density on NaCl concentration.

Surface charge density on spherical silica particles in aqueous alkali chloride solutions

Abstract: Experimental data concerning the dependence of the surface charge density of AEROSIL 300 in aqueous alkali chloride solutions on pH are analyzed under application of two different models for a spherical electrical double layer. Data for the alkali ions Li+, Na+, K+, Rb+ and Cs+ are included at five electrolyte concentrations. In the first model a counterion binding in combination with a pure diffuse spherical double layer is considered; here, a good agreement between the recalculated plots and the experimental data is found for low electrolyte concentrations (c<0.05 mol/l) only. The second model with a charge free Stern layer and without counterion binding seems to be applicable for higher electrolyte concentrations also. Furthermore the surface charge density constants from the second model agree better with those published and determined with other models or experimental methods.

Field effect measurements in doped conjugated polymer films: Assessment of charge carrier mobilities

Abstract: Conductivity and field‐effect mobility measurements using metal‐insulator‐semiconductor field‐effect‐transistor devices and acceptor density measurements using metal‐insulator‐ semiconductor (MIS) diodes are presented. The measurements were made on thin polymer films of the organic semiconductor, poly(β’‐dodecyloxy‐α,α’,‐α’,α‘terthienyl), which were doped to different conductivities using 2,3‐dichloro‐5,6‐dicyano‐1,4‐benzoquinone (DDQ) as an oxidizing agent. It is found that both the field‐effect mobility and the conductivity of these films increases superlinearly upon doping while the transistor amplification, the on/off ratio, decreases. Acceptor densities as obtained from MIS diode measurements are in close agreement with the bulk charge density as calculated from the DDQ content. However, the product of this bulk charge density, field‐effect mobility, and the elementary charge e is a factor of 100 larger than the polymer conductivity. This indicates that the average mobility for charge carriers in the...

Electron-positron Car-Parrinello methods: Self-consistent treatment of charge densities and ionic relaxations

Abstract: Rights: © 1995 American Physical Society (APS). This is the accepted version of the following article: Puska, M. J. & Seitsonen, Ari P. & Nieminen, Risto M. 1995. Electron-positron Car-Parrinello methods: Self-consistent treatment of charge densities and ionic relaxations. Physical Review B. Volume 52, Issue 15. 10947-10961. ISSN 1550-235X (electronic). DOI: 10.1103/physrevb.52.10947, which has been published in final form at http://journals.aps.org/prb/abstract/10.1103/PhysRevB.52.10947.

Microscopic formulation of Marcus’ theory of electron transfer

Abstract: A microscopic theory for the rate of nonadiabatic electron transfer is developed and its relation to classical Marcus theory is analyzed. The focus is on how the nonlinear response of a molecular solvent to a change in the charge distribution of the donor–acceptor pair influences the rate; quantum mechanical and solvent dynamical effects are ignored. Under these restrictions, the rate is determined by the probability density of the energy gap, which is defined as the instantaneous change in solvation energy upon moving an electron from the donor to the acceptor. It is shown how this probability density can be obtained from the free energies of transferring varying amounts of charge between the donor and acceptor (as specified by a charging parameter). A simple algorithm is proposed for calculating these free‐energy changes (and hence the energy gap probability density) from computer simulations on just three states: the reactant, the product, and an ‘‘anti’’‐product formed by transferring a positive unit ...

High charge density conducting polymer/graphite fiber composite electrodes for battery applications

Abstract: Novel composite electrode structures have been fabricated by single-step electropolymerization of polypyrrole onto a porous graphite fiber matrix. The graphite substrate provides a lightweight structure with high surface area. The available charge capacity of the composite electrodes was proportional to the electropolymerization time and the mass of electroactive polymer with reversible charge capacities in excess of 4.0 C/cm{sup 2} and a specific capacity of 90 mAh/g, independent of polymer mass. The rate of charge extraction was dependent on the polymer mass and the morphology of the polymer electrode. In test cells using a polypyrrole/graphite fiber anode and a polypyrrole-polystyrene sulfonate/graphite fiber cathode, the authors have demonstrated a capacity of more than 40 mAh/g based on the active mass of the undoped polymer on discharging the cell to 0.1 V over a 10 k{Omega} load. More than 70% of the available charge was extracted from the cell over 50 cycles with no degradation of cell performance.

Surface Charge and Calcium Channel Saturation in Bullfrog Sympathetic Neurons

Abstract: Currents carried by Ba2+ through calcium channels were recorded in the whole-cell configuration in isolated frog sympathetic neurons. The effect of surface charge on the apparent saturation of the channel with Ba2+ was examined by varying [Ba2+]o and ionic strength. The current increased with [Ba2+]o, and the I-V relation and the activation curve shifted to more positive voltages. The shift of activation could be described by Gouy-Chapman theory, with a surface charge density of 1 e-/140 A2, calculated from the Grahame equation. Changes in ionic strength (replacing N-methyl-D-glucamine with sucrose) shifted the activation curve as expected for a surface charge density of 1 e-/85 A2, in reasonable agreement with the value from changing [Ba2+]o. The instantaneous I-V for fully activated channels also changed with ionic strength, which could be described either by a low surface charge density (less than 1 e-/1,500 A2), or by block by NMG with Kd approximately 300 mM (assuming no surface charge). We conclude that the channel permeation mechanism sees much less surface charge than the gating mechanism. The peak inward current saturated with an apparent Kd = 11.6 mM for Ba2+, while the instantaneous I-V saturated with an apparent Kd = 23.5 mM at 0 mV. This discrepancy can be explained by a lower surface charge near the pore, compared to the voltage sensor. After correction for a surface charge near the pore of 1 e-/1,500 A2, the instantaneous I-V saturated as a function of local [Ba2+]o, with Kd = 65 mM. These results suggest that the channel pore does bind Ba2+ in a saturable manner, but the current-[Ba2+]o relationship may be significantly affected by surface charge.

Electronic band structure of sulphide spinels CuM2S4 (M=Co, Rh,Ir)

Abstract: Full-potential linearized augmented-plane-wave band calculations based on the local density approximation are performed for three sulphide spinels CuM2S4 (M=Co,Rh,Ir). The electronic states near the Fermi level consist mainly of the M ndT (n=3 for Co,4 for Rh,5 for Ir) and the S 3p orbitals. A large d gamma -d$54T splitting of the M nd bands is attributed mainly to the effects of hybridization between the M nd gamma and the S 3p orbitals. The Co 3d orbitals are more localized compared with the Rh 4d and the Ir 5d orbitals. Among the three compounds, the density of states or the partial density of states of the M n/sub /components at the Fermi level is largest in CuCo2S4. The Cu 3d orbitals form relatively narrow bands. Judging from the number of Cu 3d electrons in the muffin-tin sphere the valence of the Cu ions is Cu1+ rather than Cu2+. Hence the Cu ions are expected to be non-magnetic.

Ion condensation in the electric double layer and the corresponding Poisson-Boltzmann effective surface charge

Abstract: The effective surface charge obtained by fitting the Poisson-Boltzmann approximation to measured data is related to the actual surface charge. Evaluation of the formally exact expression, which depends upon the wall-ion direct correlation function, gives a precise measure of the amount of ion condensation in the planar double layer. Results are presented for 0.001, 0.01, and 0.1 M monovalent and divalent restricted primitive model electrolytes using the singlet hypernetted chain closure with the first bridge diagram. In general, the apparent surface charge is less than the actual surface charge, due to the influence of ion-ion correlations. Thus, the Poisson-Boltzmang approximation underestimates the actual surface charge and overestimates the amount of counterion binding. At high surface charge densities and apparent surface charge saturates, and there is a maximum surface charge density that a given electrolyte will appear to support. At surface charge densities beyond this and high concentrations (e.g. 0.1 M divalent, 75 %1* per unit surface charge), charge reversal occurs, due to overscreening by the counterions in the first layer adjacent to the surface.

Expression of the exact electron-correlation-energy density functional in terms of first-order density matrices.

Abstract: The density-functional correlation energy is given in terms of first-order density matrices. These are generated either by switching off the electron-electron interaction, or by a uniform scaling of the density. An expression using Kohn-Sham exchange-only eigenvalues is also given.

Density functional calculations of lanthanide oxides

Abstract: Density functional (DF) calculations have been performed on LaO, EuO, GdO, YbO and YbF. Gradient‐exchange and correlation functionals work satisfactorily in the outer valence shells of these molecules, but less well for the localized lanthanide f‐shells. Relativistic corrections to bond lengths, bond energies and vibrational frequencies are of quite different magnitudes and origins. The inner Ln 4f‐shell has a fractional electron population in several molecular states. We corroborate the assignment of the 0+ ground state of YbO as configuration mixed Yb2+(f14/f13s)O2−. The effective charge distribution of the lanthanide oxides is at best approximated by Ln+O−.

Cohesive, electronic and magnetic properties of the transition metal aluminides FeAl CoAl and NiAl

Abstract: Electronic structure calculations using the tight-binding linear muffin tin orbital (TB-LMTO) method have been performed for three transition metal aluminides, viz. FeAl, CoAl and NiAl. The band structures and density of states (DOS), valence electron charge density contours and Fermi surfaces have been obtained and compared with the available experimental results as well as with existing theoretical calculations. The lattice constants, cohesive energies and heat of formation at equilibrium lattice constants and bulk moduli agree with the experimental values. The calculations show varying degrees of charge transfer from Al site to the transition metal (TM) sites as one goes from FeAl to CoAl to NiAl. The magnetism of pure elements Fe, Co, Ni is mostly quenched in the stoichiometric phases, with only FeAl retaining a magnetic moment of about 0.7 mu B/atom within the framework of the LMTO.

Role of surface states for the epitaxial growth on metal surfaces

Abstract: A model is presented which relates the diffusion behavior on metal surfaces to the charge density supplied by occupied two-dimensional free-electron surface states. Depopulation of these states by confinement onto small islands or by proper adsorbates increases the diffusion barrier on flat terraces and lowers the additional barrier for step-down diffusion, thus yielding enhanced interlayer transport. This provides a possible explanation for the phenomena of low-temperature ``reentrant'' and surfactant-mediated layer-by-layer growth. The model is supported by photoemission results.