Showing papers on "Charge density published in 2003"
TL;DR: In this article, a simple polarizable water model is developed and optimized for molecular dynamics simulations of the liquid phase under ambient conditions, where the permanent charge distribution of the water molecule is represented by three point charges: two hydrogen sites and one additional M site positioned along the HOH bisector.
Abstract: A simple polarizable water model is developed and optimized for molecular dynamics simulations of the liquid phase under ambient conditions. The permanent charge distribution of the water molecule is represented by three point charges: two hydrogen sites and one additional M site positioned along the HOH bisector. Electronic induction is represented by introducing a classical charged Drude particle attached to the oxygen by a harmonic spring. The oxygen site carries an equal and opposite charge, and is the center of an intermolecular Lennard-Jones interaction. The HOH gas-phase experimental geometry is maintained rigidly and the dipole of the isolated molecule is 1.85 D, in accord with experiment. The model is simulated by considering the dynamics of an extended Lagrangian in which a small mass is attributed to the Drude particles. It is parametrized to reproduce the salient properties of liquid water under ambient conditions. The optimal model, refered to as SWM4-DP for “simple water model with four site...
643 citations
TL;DR: In this paper, the electronic energy band structure, density of states (DOS) and charge density contour of KNbO3 in the paraelectric cubic phase have been studied using the full-potential linearized augmented plane wave method within the generalized gradient approximation for exchange and correlation.
Abstract: The electronic energy band structure, density of states (DOS) and charge density contour of KNbO3 in the paraelectric cubic phase have been studied using the full-potential linearized augmented plane wave method within the generalized gradient approximation for exchange and correlation. The band structure shows an indirect (R–Γ) band gap. From the DOS analysis as well as charge density studies, we find that the bonding between K and NbO3 is mainly ionic while that between Nb and O is covalent. We have also reported results on the pressure variation of the energy gap of this compound and found that the band gap increases with increasing pressure. In order to understand the optical properties of the perovskite, the real and imaginary parts of the dielectric function, reflectivity, absorption coefficient, optical conductivity, electron energy-loss function, refractive index and extinction coefficient were calculated. The general profiles of the optical spectra were analysed and origins of the structures discussed.
370 citations
TL;DR: This work directly observed the organization of multivalent ions on cytoskeletal filamentous actin by using synchrotron x-ray diffraction and discovered an unanticipated symmetry-breaking collective counterion mechanism for generating attractions, which can fundamentally impinge on general understanding of electrostatics in aqueous media.
Abstract: Electrostatics in aqueous media is commonly understood in terms of screened Coulomb interactions, where like-charged objects, such as polyelectrolytes, always repel These intuitive expectations are based on mean field theories, such as the Poisson–Boltzmann formalism, which are routinely used in colloid science and computational biology [Israelachvili, J (1992) Intermolecular and Surface Forces (Academic, London), 2nd ed] Like-charge attractions, however, have been observed in a variety of systems [Gelbart, W M, Bruinsma, R F, Pincus, P A & Parsegian, V A (2000) Phys Today 53, 38–44] Intense theoretical scrutiny over the last 30 years suggests that counterions play a central role, but no consensus exists for the precise mechanism We have directly observed the organization of multivalent ions on cytoskeletal filamentous actin (a well defined biological polyelectrolyte) by using synchrotron x-ray diffraction and discovered an unanticipated symmetry-breaking collective counterion mechanism for generating attractions Surprisingly, the counterions do not form a lattice that simply follows actin's helical symmetry; rather, the counterions organize into “frozen” ripples parallel to the actin filaments and form 1D charge density waves Moreover, this 1D counterion charge density wave couples to twist distortions of the oppositely charged actin filaments This general cooperative molecular mechanism is analogous to the formation of polarons in ionic solids and mediates attractions by facilitating a “zipper-like” charge alignment between the counterions and the polyelectrolyte charge distribution We believe these results can fundamentally impinge on our general understanding of electrostatics in aqueous media and are relevant to a wide range of colloidal and biomedical processes
279 citations
TL;DR: In this paper, the effect of electric charge in compact stars was studied and the authors concluded that the electric fields have to be huge to have any appreciable effect on the phenomenology of the compact stars, which implies that the total charge is Coulomb.
Abstract: We study the effect of electric charge in compact stars assuming that the charge distribution is proportional to the mass density. The pressure and the density of the matter inside the stars are large, and the gravitational field is intense. This indicates that electric charge and a strong electric field can also be present. The relativistic hydrostatic equilibrium equation, i.e., the Tolman-Oppenheimer-Volkoff equation, is modified in order to include electric charge. We perform a detailed numerical study of the effect of electric charge using a polytropic equation of state. We conclude that in order to see any appreciable effect on the phenomenology of the compact stars, the electric fields have to be huge $(\ensuremath{\sim}{10}^{21}$ V/m), which implies that the total charge is $Q\ensuremath{\sim}{10}^{20}$ Coulomb. From the local effect of the forces experienced on a single charged particle, it is expected that each individual charged particle is quickly ejected from the star. This in turn produces a huge force imbalance, and the gravitational force overwhelms the repulsive Coulomb and fluid pressure forces. The star can then collapse to form a charged black hole before all the charge leaves the system.
230 citations
TL;DR: In this paper, the dopant density and the bulk charge-carrier mobility can simultaneously be estimated from the transfer characteristics of a single disordered organic transistor, by using a concentric device geometry.
Abstract: We demonstrate that, by using a concentric device geometry, the dopant density and the bulk charge-carrier mobility can simultaneously be estimated from the transfer characteristics of a single disordered organic transistor. The technique has been applied to determine the relation between the mobility and the charge density in solution-processed poly(2,5-thienylene vinylene) and poly(3-hexyl thiophene) thin-film field-effect transistors. The observation that doping due to air exposure takes place already in the dark, demonstrates that photoinduced oxygen doping is not the complete picture.
214 citations
TL;DR: In this article, the authors measured the density of charged sites in an agarose gel as a function of added salt and pH and evaluated the Donnan potential, finding that the majority of binding sites in the gel are pyruvate groups, with an intrinsic protonation constant of log Kaint = 3.9.
Abstract: The nature and density of charged sites in an agarose gel have been studied. Diffusion and partition coefficients of various organic and inorganic ions have been measured as a function of ionic strength, μ, and pH to investigate the solute−gel interactions resulting from charge effects and specific complexation. The majority of binding sites in the gel are pyruvate groups, with an intrinsic protonation constant of log Kaint = 3.9. We measured the charge density of the gel as a function of added salt and pH and evaluated the Donnan potential. The partition coefficients of cations decrease and those of anions increase with increasing ionic strength because of progressive screening of the anionic sites in the gel, as predicted by the Boltzmann and Poisson−Boltzmann equations. The charges in the gel become completely screened at μ ≈ 10-2. As predicted with the Smoluchowski−Poisson−Boltzmann theory, the diffusion coefficient of cationic species is reduced at low ionic strength. We tested both cylindrical and s...
194 citations
TL;DR: In this article, a more detailed analysis revealed that the circulation speed in a pure vacuum gap is too high when compared with observations, and some pul- sars demonstrate significant time variations of the drift rate, including a change of the apparent drift direction.
Abstract: The classical vacuum gap model of Ruderman & Sutherland, in which spark-associated subbeams of subpulse emission circulate around the magnetic axis due to the E × B drift of spark plasma filaments, provides a natural and plausible physical mechanism of the subpulse drift phenomenon. Moreover, this is the only model with quantitative predictions that can be compared with observa- tions. Recent progress in the analysis of drifting subpulses in pulsars has provided a strong support to this model by revealing a number of subbeams circulating around the magnetic axis in a manner compatible with theoretical predictions. However, a more detailed analysis revealed that the circulation speed in a pure vacuum gap is too high when compared with observations. Moreover, some pul- sars demonstrate significant time variations of the drift rate, including a change of the apparent drift direction, which is obviously inconsistent with the E × B drift scenario in a pure vacuum gap. We attempted to resolve these discrepancies by considering a partial flow of iron ions from the positively charged polar cap, co- existing with the production of outflowing electron-positron plasmas. The model of such charge-depleted acceleration region is highly sensitive to both the critical ion temperature Ti � 10 6 K (above which ions flow freely with the corotational charge density) and the actual surface temperature Ts of the polar cap, heated by the bombardment of ultra-relativistic charged particles. By fitting the obser- vationally deduced drift-rates to the theoretical values, we managed to estimate polar cap surface temperatures in a number of pulsars. The estimated surface temperatures Ts correspond to a small charge depletion of the order of a few percent of the Goldreich-Julian corotational charge density. Nevertheless, the re-
183 citations
TL;DR: In this article, the Kohn-Sham equations are solved with periodic boundary conditions and within the all-electron projector-augmented-wave (PAW) formalism, using a generalized gradient approximation to account for electron exchange and correlation.
162 citations
TL;DR: In this article, a residue-based protein−surface interaction potential model was developed to study the adsorption and orientation of two antibodies, IgG1 and IgG2a, by Monte Carlo simulations.
Abstract: In this work, we develop a new residue-based protein−surface interaction potential model. With this model, the adsorption and orientation of two antibodies, IgG1 and IgG2a, are studied by Monte Carlo simulations. Effects of surface charge density and sign, and solution ionic strength are examined in our simulations. Simulation results show that van der Waals and electrostatic interactions codetermine the orientation of adsorbed antibodies. At low surface charge density and high solution ionic strength, where van der Waals interactions dominate, both IgG1 and IgG2a exhibit multiple orientations. At high surface charge density and low solution ionic strength, where electrostatic interactions dominate, there are preferred orientations for these two antibodies on both positively and negatively charged surfaces, which are verified by experimental results. Due to a smaller dipole moment, IgG2a has more possible orientations than IgG1. IgG1 adsorbed on a positively charged surface shows an “end-on” orientation, ...
144 citations
TL;DR: In this article, the lattice properties and electronic structure of graphite and graphite lattices were investigated using the local density approximation (LDA) and generalized gradient approximation (GGA).
Abstract: We calculate the lattice properties and electronic structure of graphite and ${\mathrm{LiC}}_{6}$ within the most widely used density-functional theory implementation, the local density approximation (LDA). Improvements to the LDA in the form of a generalized gradient approximation (GGA) are explored. Structural parameters predicted by the LDA, as expected, underestimate experiment within a 1%--2% margin of accuracy. The GGA does not give a good account in the prediction of lattice parameter c, especially in graphite, although it does give a reliable description of ${\mathrm{LiC}}_{6}.$ The effect on intercalating lithium into graphite, where charge transfer from lithium to carbon layers (graphenes) is expected, is discussed from the valence charge density, partial density of states, and energy band structure plots. The latter plot is also compared with inelastic neutron scattering results and low-energy electron diffraction results. We extend this work by calculating the elastic constants and bulk modulus for both graphite and ${\mathrm{LiC}}_{6}$ structures. These results are in excellent agreement with the available experimental data. The calculated hydrostatic pressure dependence of the crystal structures is also found to be in good agreement with the results of high-resolution x-ray structural studies and with other experimental data as well as with other calculations. The analysis of electronic structure at 0 GPa (ambient pressure) is used to resolve inconsistencies between previous LDA calculations.
136 citations
TL;DR: In this paper, the effect of mutual polyelectrolyte charge and secondary interactions on the formation of multilayer films of a lowly charged copolymer of acrylamide and poly(acrylic acid) (PAA) was investigated.
Abstract: We report on the effect of mutual polyelectrolyte charge and secondary interactions on the formation of polyelectrolyte multilayer films of a lowly charged copolymer of acrylamide and [3-(2-methylpropionamido)propyl]trimethylammonium chloride, AM-MAPTAC 10 (10 mol % of cationic monomers), and poly(acrylic acid) (PAA). These multilayer films were constructed on planar substrates by the sequential adsorption of AM-MAPTAC 10 and PAA at different pH conditions. Surface plasmon resonance spectroscopy (SPRS), quartz crystal microbalance (QCM), and atomic force microscopy (AFM) were employed to follow the film formation process. While AM-MAPTAC 10 is permanently charged, the ionization of PAA is adjusted by changing the pH of the adsorption solutions. In contrast with previous studies, it is shown that multilayer formation with one polyelectrolyte with a charge density of 10 mol % is possible. However, the multilayer growth follows a different pattern depending on the relative charge density of PAA. At low pH, w...
TL;DR: Variation of charge density properties and derived local energy densities are investigated, over all inter- and intramolecular interactions present in altogether five complexes of DMAN.
Abstract: Experimental charge density distributions in a series of ionic complexes of 1,8-bis(dimethylamino)naphthalene (DMAN) with four different acids: 1,2,4,5-benzenetetracarboxylic acid (pyromellitic acid), 4,5-dichlorophthalic acid, dicyanoimidazole, and o-benzoic sulfimide dihydrate (saccharin) have been analyzed. Variation of charge density properties and derived local energy densities are investigated, over all inter- and intramolecular interactions present in altogether five complexes of DMAN. All the interactions studied {[O···H···O]-, C−H···O, [N−H···N]+, O−H···O, C−H···N, Cπ···Nπ, Cπ···Cπ, C−H···Cl, N−H+ } follow exponential dependences of the electron density, local kinetic and potential energies at the bond critical points on the length of the interaction line. The local potential energy density at the bond critical points has a near-linear relationship to the electron density. There is also a Morse-like dependence of the laplacian of rho on the length of interaction line, which allows a differentiat...
TL;DR: It is predicted that substantial enhancement of sensitivity, selectivity, and reliability of microarrays can be achieved by control of the surface conditions, especially at low added salt or short linker length.
Abstract: We present a theoretical thermodynamic framework for the design of more efficient oligonucleotide microarrays. A general thermodynamic relation is derived to describe the electrostatic surface effects on the binding of the assayed biomolecule to a surface-tethered molecular probe. The relation is applied to analyze how the nucleic acid target, the oligonuleotide probe, and their DNA duplex electrostatic interactions with the surface affect the hybridization on DNA arrays. Taking advantage of a closed form exact solution of the linear Poisson-Boltzmann equation for a charged ion-penetrable sphere in electrolyte solution interacting with a plane wall, we study the effects of the surface and solution conditions. Binding free energy is found as a function of the surface material, dielectric or metal, the surface charge density, linker molecule length, temperature, and added salt content. The charge or electric potential of the dielectric or metal surface, respectively, is shown to dominate the hybridization, especially at low added salt or short linker length. We predict that substantial enhancement of sensitivity, selectivity, and reliability of microarrays can be achieved by control of the surface conditions. As examples, we discuss how to overcome two limitations of current technologies: nonequal sensitivity of the probes with different GC and AT bases content, and poor match/mismatch discrimination. In addition, we suggest the design of microarray conditions where the tested nucleic acid is unfolded, thus making possible the screening of a larger sequence with single nucleotide resolution. These promising findings are discussed and further experimental tests suggested.
TL;DR: A self-consistent tight-binding model confirms the observed molecular Faraday cage effect, i.e., most of the charge resides on the outer wall, even when this wall was originally semiconducting and the inner wall was metallic.
Abstract: A double-walled carbon nanotube is used to study the radial charge distribution on the positive inner electrode of a cylindrical molecular capacitor. The outer electrode is a shell of bromine anions. Resonant Raman scattering from phonons on each carbon shell reveals the radial charge distribution. A self-consistent tight-binding model confirms the observed molecular Faraday cage effect, i.e., most of the charge resides on the outer wall, even when this wall was originally semiconducting and the inner wall was metallic.
TL;DR: In this paper, a new measurement system is described for measuring the bipolar charge distribution, which consists of a vertical array of seven Faraday pail sensors, which can selectively detect different charge components based upon particle size and charge repulsion.
Abstract: It is well known that many common industrial powder handling operations such as pneumatic transport, mixing, and fluidization may produce a net charge on the powder particles. However, it is less well known that the net charge is often the result of a bipolar charge distribution in which the smaller particles acquire charges of a polarity opposite to those on the larger. This suggests contact charging between particles having the same chemical makeup. Very little quantitative data exist in the literature concerning this observation and no acceptable explanation currently exists. The purpose of this paper is to review the previous published work and to describe some results showing bipolar charging using polymer powders in fluidized beds. A new measurement system is described for measuring the bipolar charge distribution. This consists of a vertical array of seven Faraday pail sensors, which can selectively detect different charge components based upon particle size (gravity segregation) and charge (space-charge repulsion). For the experiments reported here the charge and mass values were measured for each sensor allowing the calculation of charge-to-mass ratio (Q/M). In addition, size distribution and surface analyses were carried out for representative samples of the powder components. Data are presented for several types of polymer powders (surface area mean diameter <100 /spl mu/m). The results show that, for a given powder, even though the net charge may be positive or negative, the smaller particles show a negative charge and the coarser particles positive. These results are compared under several possible hypotheses. Each of these possibilities is examined using the measurements of the Q/M for different size fractions and the results of surface analysis and particle size distributions of these fractions.
TL;DR: The effect of polymer charge density, polyelectrolyte concentration, and ionic strength on the formation of multilayer films by sequential adsorption of a polyanion (poly(styrene sulfonate), PSS) was studied in this paper.
Abstract: The effect of polymer charge density, polyelectrolyte concentration, and ionic strength on the formation of multilayer films by sequential adsorption of a polyanion (poly(styrene sulfonate), PSS) a...
TL;DR: In this paper, a model polycation/polyanion multilayer system, poly(allylamine hydrochloride) (PAH), and an azobenzene-containing polyelectrolyte (P-Azo) prepared at varying charge densities were studied by nano-inden- tation experiments using an atomic force microscope.
Abstract: Mechanical properties of polyelectrolyte multilayer thin films were studied by nanoinden- tation experiments using an atomic force microscope. We obtained force-distance measurements for a model polycation/polyanion multilayer system, poly(allylamine hydrochloride) (PAH), and an azobenzene- containing polyelectrolyte (P-Azo) prepared at varying charge densities. The relative Young's modulus of the films was determined as a function of the ionization fraction of the multilayer films, prepared to have identical thickness. Multilayer films assembled with polyelectrolytes of high charge density exhibited an elastic modulus that was significantly larger (nearly 2 orders of magnitude) than those prepared with low charge density. An estimate of the relative loop length between "ionic cross-links" in the multilayer films is then ascertained by analogy to previously studied covalently cross-linked polymer networks. The modulus values in our films ranged from 10-2 to 10-4 kPa, and this implies loop lengths of 1-50 segments. Atomic force microscopy force-distance measurements were also used to compare the relative adhesion values between polycation and polyanion layers in films constructed with varying charge densities. This was done by coating an AFM tip with multilayers and indenting into a surface containing the same multilayer film but capped with the oppositely charged surface polyelectrolyte layer. Adhesion values were typically between 0.5 and 6.7 nN and were found to depend on the ionic cross-link density of the PAH/P-Azo film, in which the highly ionically cross-linked samples exhibited the largest adhesion.
TL;DR: This work studied multiple ionization of single nucleobases by means of slow highly charged ions (Xe(q+), q=5-25) to shed light on the charge distribution within the molecule and on the fragmentation dynamics of these intermediate size systems.
Abstract: We studied multiple ionization of single nucleobases by means of slow highly charged ions (${\mathrm{X}\mathrm{e}}^{q+}$, $q=5--25$). The products of the subsequent fragmentation were studied using high resolution coincidence time-of-flight spectrometry. We observed a strong dependence of the fragment kinetic energies on the initial charge state of the intermediate parent ions as well as on the initial chemical environment of the respective fragment ions within the parent molecule. The data allow us to shed light on the charge distribution within the molecule as well as on the fragmentation dynamics of these intermediate size systems.
TL;DR: It is argued, on the basis of density functional calculations, that a self-assembled monolayer of oligo(ethylene glycol) or n-alkanes in contact with water will preferentially adsorb hydroxyl ions on both methoxy- and hydroxide-terminated endgroups, thus charging the surface region of the SAM negatively with an estimated charge density of about 1 microC/cm(2) in agreement with recent experiments.
Abstract: It is argued, on the basis of density functional calculations, that a self-assembled monolayer of oligo(ethylene glycol) or n-alkanes in contact with water will preferentially adsorb hydroxyl ions (either from autoionization of water or added to the solution) on both methoxy- and hydroxide-terminated endgroups, thus charging the surface region of the SAM negatively with an estimated charge density of about 1 μC/cm2 in agreement with recent experiments. The negative charging can explain long-ranged forces between opposing SAM surfaces. On dense SAMs, hydroxyl ions are highly mobile. Hydronium ions can absorb by penetration into the SAM provided there is enough lateral space for their encapsulation. The important role of hydration is demonstrated by calculating the excess binding energy of adsorption using a Born−Haber cycle.
TL;DR: In this paper, the internal electric field in multilayer organic light-emitting diodes (OLEDs) is investigated using a combination of experimental measurement and numerical device modeling.
Abstract: The internal electric field in multilayer organic light-emitting diodes (OLEDs) is investigated using a combination of experimental measurement and numerical device modeling. This approach results in a detailed understanding of the functioning of a multilayer OLED. The method is applied to a standard device structure that has received broad attention in literature. From the experimental part, we have demonstrated that the average electric field inside the hole transport layer is larger than or equal to the average field in the emission layer over the entire current range. Device simulations fully clarify the situation, giving an insight into the space charge effects as well as the hole and the electron current distributions in the device. In particular, we find that there is a leakage of unrecombined holes towards the cathode at low voltages. We also find a strong variation of the electric field in the tris(8-hydroxyquinolinato)aluminum layer.
TL;DR: The interpretation of the entropy sum as a correlation measure is demonstrated for isoelectronic series via an analytical expression that models the asymptotic behavior of the electronic charge density in position space and the cusp behavior in momentum space as discussed by the authors.
Abstract: The interpretation of the entropy sum as a correlation measure is demonstrated for isoelectronic series via an analytical expression that models the asymptotic behavior of the electronic charge density in position space and the cusp behavior in momentum space We also develop an expression for the entropy sum in neutral atoms with an explicit dependence on the ionization energy and the atomic number The results obtained from these relations are in qualitative agreement with the behavior observed from ab initio calculations A connection between the entropy sum and the correlation energy is obtained for the weakly inhomogeneous electron gas and demonstrated via calculations for the helium isoelectronic series
TL;DR: The charge density refinement of the protein was performed with the program mopro by using the transferability principle and the database of charge density parameters built from crystallographic analyses of peptides and amino acids, and the potentials obtained directly from the database are in excellent agreement with the experimental ones.
Abstract: The electron density and electrostatic potential in an Aldose Reductase holoenzyme complex have been studied by DFT and diffraction methods. Aldose reductase is involved in the reduction of glucose in the polyol pathway using NADPH as a co-factor. The ultra-high resolution of the diffraction data and the low thermal displacement parameters of the structure allow accurate atomic positions and an experimental charge density analysis. Based on the X-ray structural data, order-N Density Functional Theory (DFT) calculations have been performed on 711 atoms in the active site of the molecule. The charge density refinement of the protein was performed with program MoPro using the transferability principle and our database of charge density parameters built from crystallographic analyses of peptides and amino acids.
TL;DR: In this article, the spin-electric coupling was employed for noninvasive single-spin detection by electrical methods, and a spin relaxation mechanism due to long-range coupling to electrons in gates and elsewhere in the system, and the contribution comparable to and in some cases dominant over, previously discussed mechanisms.
Abstract: Due to the spin-orbital coupling in an anisotropic semiconductor quantum dot, a freely precessing electron spin produces a time-dependent charge density. This creates a sizable electric field outside the dot, leading to promising applications in spintronics. The spin-electric coupling can be employed for noninvasive single-spin detection by electrical methods. We also consider a spin relaxation mechanism due to long-range coupling to electrons in gates and elsewhere in the system, and find a contribution comparable to, and in some cases dominant over, previously discussed mechanisms.
TL;DR: In this article, the solvent effects on the geometrical and electronic structures, as well as one-and two-photon absorption processes, of a newly synthesized asymmetrical charge transfer (CT) twophoton absorbing (TPA) organic molecule were investigated.
Abstract: Time-dependent hybrid density functional theory in combination with polarized continuum model has been applied to study the solvent effects on the geometrical and electronic structures, as well as one- and two-photon absorption processes, of a newly synthesized asymmetrical charge-transfer (CT) two-photon absorption (TPA) organic molecule. The TPA cross section calculated from a generalized two-state model and solvatochromic shift of the CT state are found to be solvent dependent, for which a nonmonotonic behavior with respect to the polarity of the solvents has been observed. The calculated properties are in good agreement with the experimental data available. The character of the CT state is visualized by plotting its charge density difference from ground state, in which an excess of electron density on the donor side of the molecule is found. This implies that the excited molecule is ready to donate its electron to the surroundings. The energetic aspect of the electron donation is discussed by examining the solvent dependence of the molecular ground state oxidation potential. The importance of the electron correlation for describing the two-photon absorption is also demonstrated.
TL;DR: In this paper, a model of cross-linked polyelectrolyte gels was investigated by means of Monte Carlo simulations, which contained a charged defect-free th ee-dimensional network of a diamond-like topology and explicit counterions.
Abstract: Model systems of cross-linked polyelectrolyte gels were investigated by means of Monte Carlo simulations. The model contained a charged defect-free th ee-dimensional network of a diamond-like topology and explicit counterions. Pressure vs volume relations and chain extensions were determined and compared to those of the corresponding polyelectrolyte solution. The structure of the gel was characterized by radial distribution functions. In the swollen state, the network particles and the counterions are inhomogeneously distributed in space. Also, the properties of the polyelectrolyte gels were investigated at different charge density, cross-linking density, chain flexibility, and counterion valence. An increase in the gel volume was observed for increasing charge density, decreasing cross-linking density, and increasing chain stiffness. The exchange of the monovalent counterions for divalent counterions reduced the equilibrium volume of the gel substantially. The affine assumption, which states that a linear relation exists between chain end-to-end separation and macroscopic gel size, was found to be only of limited validity, whereas the Gaussian chain appr ximation was never fulfilled for polyelectrolyte gels. (Less)
TL;DR: In this paper, the average solvent electrostatic potential/molecular dynamics (ASEP/MD) and the free-energy gradient methods are applied together with the multidimensional geometry optimization of molecules in solution.
Abstract: The average solvent electrostatic potential/molecular dynamics (ASEP/MD) and the free-energy gradient methods are applied together with the multidimensional geometry optimization of molecules in solution. The systems studied were formamide in aqueous solution and water and methanol in liquid phase. The solute molecules were described through ab initio quantum mechanics methods (density dunctional theory or Moller–Plesset second order perturbation theory) while the solvent structure was obtained from Molecular Dynamics calculations. The method is very efficient; the increase in computation time is minimal with respect to previous ASEP/MD versions that worked at a fixed geometry. Despite the use of the mean field approximation in the calculation of the solvent reaction potential the agreement with previous theoretical calculations was satisfactory. Large changes were observed in the solute charge distribution induced by the solvent, and the solute polarization was accompanied by an increase in the solvent s...
TL;DR: In this article, the augmented plane-wave plus local-orbitals method is applied to investigate the charge density of Cu 2 O by means of Bader's topological analysis and electric-field gradients (EFGs).
Abstract: We have applied the augmented plane-wave plus local-orbitals method to investigate the charge density of Cu 2 O by means of Bader's topological analysis and electric-field gradients (EFGs). It is rather clear that a simpleCu + -O 2 - model is inadequate for the explanation of bonding properties of Cu 2 O. Appearance of s-d hybridization in this system has been pointed out already in the literature. However, the amount of charge transferred from d to s Cu orbitals is overestimated by both the local-density approximation (LDA) and the generalized gradient approximation. As a result, the calculated EFG is underestimated by about 50% compared to experiment. Also the topological analysis of calculated densities suggests that density-functional theory overestimates the covalency of the Cu-O bond. In order to demonstrate the role of s-d hybridization as the main reason for the mentioned discrepancies, we have utilized artificially modified basis sets. Removing part of the 4s Cu character from the wave function increases the absolute EFGs and decreases the calculated bulk modulus, both resulting in better agreement with experiment. This is a result of an increased Cu d occupation and consequently a decreased asphericity of the d charge distribution. A more physical description of the localized nature of the 3d orbitals should be given by the LDA+ U correction. We find that LDA+ U "self-interaction corrected" (SIC) by Anisimov et al. [V.I. Anisimov, I.V. Solovyev, M.A. Korotin, M.T. Czyzyk, and G.A. Sawatzky, Phys. Rev. B 48, 16929 (1993)] leads to EFGs in good agreement with experiment while LDA+U "around mean field" Cyzyk and Sawatzky [Phys. Rev. B 49, 14211 (1994)] even worsens the results. This is to be expected according to the analysis given above, which indicates that the on-site correlations are important but are implemented only in the LDA+ U (SIC) method.
TL;DR: In this paper, the topology of the charge density of β-chalcogenvinylaldehydes has been investigated at the B3LYP/6-311 + 6-311+G(3df,2p) level.
Abstract: Density functional calculations, at the B3LYP/6-311 + G(3df,2p) level, have been carried out for the complete series of β-chalcogenvinylaldehydes to investigate whether the topology of the charge density of these systems can provide some useful information on the strength of X–H⋯Y or X⋯H–Y intramolecular hydrogen bonds or on the strength of H–X⋯Y or X⋯Y–H intramolecular chalcogen–chalcogen interactions. We have shown that, in general, there exist good linear correlations between the charge density at the ring critical point and (a) the relative strength of these interactions as measured by appropriate homodesmotic reactions, (b) the X⋯Y chalcogen–chalcogen distance. It must be emphasized, however, that while the latter correlation is systematically fulfilled, the former is only observed when the relative strength of the intramolecular interaction, either a IHB or a chalcogen–chalcogen interaction, varies inversely with the chalcogen–chalcogen distance. Also importantly, the variation of the charge density at the ring critical point correlates both, with the change in the stability of the system and with the change in the chalcogen–chalcogen distance, when the IHB is replaced by the chalcogen–chalcogen interaction, through an internal rotation of the X–H or the Y–H group.
01 Dec 2003
TL;DR: In this article, the electrostatics of nanowire transistors are studied by solving the Poisson equation selfconsistently with the equilibrium carrier statistics of the nanowires.
Abstract: The electrostatics of nanowire transistors are studied by solving the Poisson equation self-consistently with the equilibrium carrier statistics of the nanowire. For a one-dimensional, intrinsic nanowire channel, charge transfer from the metal contacts is important. We examine how the charge transfer depends on the insulator and the metal/semiconductor Schottky barrier height. We also show that charge density on the nanowire is a sensitive function of the contact geometry. For a nanowire transistor with large gate underlaps, charge transferred from bulk electrodes can effectively "dope" the intrinsic, ungated region and allow the transistor to operate. Reducing the gate oxide thickness and the source/drain contact size decreases the length by which the source/drain electric field penetrates into the channel, thereby, improving the transistor characteristics.
TL;DR: This work examines how the results of ab initio electronic structure calculations, based upon planewave DFT methods, on general configurations of ions derived from simulations at finite temperature, may be used to parameterize an "aspherical ion method" (AIM) potential.
Abstract: Oxide potentials which transfer well between different materials have to account explicitly for many-body contributions to the interaction potentials between the ions. These include dipole and quadrupole polarization effects and the compression and deformation of an oxide ion by its immediate coordination environment. Such complex potentials necessarily involve many parameters. We examine how the results of ab initio electronic structure calculations, based upon planewave DFT methods, on general configurations of ions derived from simulations at finite temperature, may be used to parameterize an “aspherical ion method”
(AIM) potential (A. J. Rowley, P. emmer, M. Wilson and P. A. Madden, J. Chem. Phys., 1998, 108, 10 209). Dipoles and quadrupoles on the individual ions are obtained via a transformation of the Kohn–Sham orbitals to localized orbitals on each ion, which enables a distorted charge density for each ion to be obtained. The dipoles and quadrupoles appearing in polarization parts of the AIM potential are fit to those obtained from the ab initio ionic charge densities obtained in this way. The remaining parts of the potential, describing short-range repulsive interactions between ions with compressed and deformed charge densities, are fit to the ab initio forces and the stress tensor. By using a sufficiently large and varied set of configurations on which to carry out this optimization, an excellent transferable potential is obtained.