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Dust impacts on spacecraft are commonly detected by antenna instruments as transient voltage perturbations. The signal waveform is generated by the interaction between the impact-generated plasma cloud and the elements of the antenna-spacecraft system. A general electrostatic model is presented that includes the two key elements of the interaction, namely the charge recollected from the impact plasma by the spacecraft and the fraction electrons and cations that escape to infinity. The clouds of escaping electrons and cations generate induced signals, and their vastly different escape speeds are responsible for the characteristic shape of the waveforms. The induced signals are modeled numerically for the geometry of the system and the location of the impact. The model employs a Maxwell capacitance matrix to keep track of the mutual interaction between the elements of the system. A new reduced-size model spacecraft is constructed for laboratory measurements using the dust accelerator facility. The model spacecraft is equipped with four antennas: two operating in a monopole mode, and one pair configured as a dipole. Submicron-sized iron dust particles accelerated to > 20 km/s are used for test measurements, where the waveforms of each antenna are recorded. The electrostatic model provides a remarkably good fit to the data using only a handful of physical fitting parameters, such as the escape speeds of electrons and cations. The presented general model provides the framework for analyzing antenna waveforms and is applicable for a range of space missions investigating the distribution of dust particles in relevant environments., Manuscript accepted online by JGR: Space Physics on 13 August 2021

This paper attempts to propose a model, called the electrostatic model of homeopathy, to explain a mechanism for the physicochemical activities of highly diluted homeopathic medicines (HMs). According to this proposed model, the source of HMs' action is dipole orientations as electrostatic imprints of the original molecules carried by diluent molecules (such as sugar molecules) or potentization-induced aqueous nanostructures. The nanoscale domains' contact charging and dielectric hysteresis play critical roles in the aqueous nanostructures' or sugar molecules' acquisition of the original molecules' dipole orientations. The mechanical stress induced by dynamization (vigorous agitation or trituration) is a crucial factor that facilitates these phenomena. After dynamization is completed, the transferred charges revert to their previous positions but, due to dielectric hysteresis, they leave a remnant polarization on the aqueous nanostructures or sugar molecules' nanoscale domains. This causes some nanoscale domains of the aqueous nanostructures or sugar molecules to obtain the original substance molecules' dipole orientations. A highly diluted HM may have no molecule of the original substance, but the aqueous nanostructures or sugar molecules may contain the original substance's dipole orientations. Therefore, HMs can precisely aim at the biological targets of the original substance molecules and electrostatically interact with them as mild stimuli.

We report the synthesis of the lanthanide-(bis)boryloxide complex [Dy{OB(NArCH)2}2(THF)4][BPh4] (2Dy, Ar = 2,6-Pri2C6H3), with idealised D4h@Dy(III) point-group symmetry. Complex 2Dy exhibits single-molecule magnetism (SMM), with one of the highest energy barriers (Ueff = 1565(298) K) of any six-coordinate lanthanide-SMM. Complex 2Dy validates electrostatic model predictions, informing the future design of lanthanide-SMMs.

Wire-mesh sensors (WMSs) have been used to measure various characteristics of bubbles, such as volume fraction, size and velocity. The WMS used in this study consists of a grid of nine by nine wires arranged in an orthogonal shape. Transmitter wires are activated sequentially and receiver wires pick up the electric signals and convert them to a current. Multiphase flow, the electric conductivity of each phase, allows characteristic electrical signals to flow and from this information the volume fraction can be calculated. However, deformation of bubbles and the intrusive effects of wires have yet to be considered. To address these issues, we conducted numerical simulations using commercial software. The actual behavior of a single bubble was considered to calculate the electrostatic and two-phase flow simultaneously. First, a bubble was considered stationary in a spherical shape at the center of the WMS. A user-defined function was used to calculate the governing equation of the electric field and then activate electric signals on the transmitter wires. The numerical results were compared with reference values, and the error of a volume fraction was quantified. After that, we considered a bubble rising without a change of shape in the pseudo-dynamic process. However, for the dynamic process, a volume of fluid model was added and a bubble interface that reflects its actual movement was considered. A comparison between the pseudo-dynamic and dynamic processes was conducted using the same positional relations between the wire and bubble surface. When the bubble contacted with and then separated from the wire, differences in the received signal and an overshooting phenomenon were observed owing to deformation of the bubble. Also, the range of locations where the bubble would begin to affect the electric field because of the flattened shape of the bubble was observed.

A novel apparatus was designed, built and tested to study the magnitude and direction of charge transfer in particle-particle collisions of insulator particles. This design ensured that the measured change in the charge of each particle was solely due to the binary collision between the particles. The collisions were confirmed by monitoring the particle trajectories using a high frame-rate camera coupled with particle-tracking software. This apparatus can aid in understanding the charge transfer mechanisms behind particle-particle charge transfer, leading to the development of an electrostatic model to simulate particle-particle contact charging in gas-solid flows. Upon preliminary testing of this apparatus with 3.18 mm diameter polytetrafluoroethylene (PTFE) particles, it was confirmed that triboelectrification solely due to the particle-particle collision can be observed. It was found that, if the particles carry low charges relative to their charge saturation point, their initial charge before collision does not dictate the direction of charge transfer.

The ion–molecular model of water, which assumes a high concentration of short-lived ions of Н3О+ and ОН– and their proton exchange with neutral Н2О molecules, indicates that water cohesion is achieved in the form of electrostatic interaction between oscillating ions and the molecular dipoles that surround them. It is shown that these oscillators with a frequency of 5 THz are responsible for the specific heat capacity of water.

A study of the strong N-X⋅⋅⋅- O-N+ (X=I, Br) halogen bonding interactions reports 2×27 donor×acceptor complexes of N-halosaccharins and pyridine N-oxides (PyNO). DFT calculations were used to investigate the X⋅⋅⋅O halogen bond (XB) interaction energies in 54 complexes. A simplified computationally fast electrostatic model was developed for predicting the X⋅⋅⋅O XBs. The XB interaction energies vary from -47.5 to -120.3 kJ mol-1 ; the strongest N-I⋅⋅⋅- O-N+ XBs approaching those of 3-center-4-electron [N-I-N]+ halogen-bonded systems (ca. 160 kJ mol-1 ). 1 H NMR association constants (KXB ) determined in CDCl3 and [D6 ]acetone vary from 2.0×100 to >108 m-1 and correlate well with the calculated donor×acceptor complexation enthalpies found between -38.4 and -77.5 kJ mol-1 . In X-ray crystal structures, the N-iodosaccharin-PyNO complexes manifest short interaction ratios (RXB ) between 0.65-0.67 for the N-I⋅⋅⋅- O-N+ halogen bond.

We propose a reinforced quasi-random forest for classification task. Reinforcement is performed iteratively by adding new trees to the forest. Our method assigns an importance to each of the attributes and identifies the attributes that causes the mis-classification of data points during training. The new trees are constructed using the mis-classified data points with reduced set of attributes. The attributes for splitting the nodes of the reinforced trees are found in a deterministic manner. Hence the new trees are quasi-random in nature. The best out of all the new trees are found using a novel electrostatic model. These trees are termed as reinforced trees. Additions of reinforced trees to the existing forest ensure maximum reduction in classification error. The efficacy of the proposed method is established through experiments on breast cancer datasets for detecting mitotic nuclei. Results of our method show significant improvement compared to other state-of-the-art approaches. Results on benchmark datasets show as much as 14% reduction in classification error.

The MUlti-start optimization algorithm for Surface complexation Equilibrium (MUSE) algorithm has been developed to optimize the fitting of thermodynamic constants for surface complexation modeling (SCM). Although there is a plethora of software to perform data fitting and determine intrinsic equilibrium constants, the algorithms used are highly dependent on initial values and choice of parameters. This limits their transferability to model other systems, for example, reactive transport processes. With this in mind, a hybridized optimization approach, based on a multistart algorithm combined with a local optimizer, has been developed to allow the simultaneous optimization of SCM parameters and to assess the sensitivity of these parameters to changes in the model assumptions. In this study, the CD–MUSIC formalism with a Basic Stern electrostatic model is utilized to model chromate adsorption on ferrihydrite, although the MUSE algorithm can be applied to any adsorption data set and be implemented in any mode...

This chapter deals with the facial selectivity of nucleophilic additions to carbonyl compounds. This is explained using models such as the Cram’s model, Anh–Felkin modification of Cram’s model, Houk’s transition structure model, Houk’s electrostatic model, Cieplak’s σ → σ*# model, and cation coordination model. The intricacies, variations, and predicted selectivities of these models are elaborated with examples. It has been argued that the Cieplak’s σ → σ*# model is applicable to only those reactions that proceed through product-like transition structures. Using the cation coordination model, the facial selectivities of a number of substrates, including the better anti-selectivity of endo,endo-2,3-diethyl-7-norbornanone in comparison to that of endo,endo-2,3-dimethyl-7-norbornanone, have been convincingly explained.

The structural and electronic properties of ${\mathrm{KTaO}}_{3}$/${\mathrm{KZnF}}_{3}$ and ${\mathrm{KTaO}}_{3}$/${\mathrm{KNiF}}_{3}$ oxyfluoride superlattices are studied from first-principles density functional theory calculations. We highlight, that beyond a critical layer thickness, these systems exhibit an insulator to metal transition that gives rise to the appearance of two-dimensional electron and hole gas, confined both, due the band alignment, within the oxide layer. The origin of the insulator to metal transition is related to the polar discontinuity at the interfaces. The behavior is discussed in terms of a simple electrostatic model and compared to that of the prototypical ${\mathrm{LaAlO}}_{3}$/${\mathrm{SrTiO}}_{3}$ oxide system. The magnetic properties ${\mathrm{KTaO}}_{3}$/${\mathrm{KNiF}}_{3}$ superlattices are further discussed, revealing a sizable Rashba-type spin splitting at these interfaces, much larger than in similar oxide/oxide systems.

Small $\Gamma-L$ valley energy offset in III-V materials such as GaAs is shown to improve the transistor performance, overcoming the density of states bottleneck while maintaining high injection velocity. Existing compact models for III-V channel transistors only account for the dominant $\Gamma$ -valley charges. This underestimates the transistors circuit performance. In this work, for the first time, a compact model is introduced to include the $L$ -valley charges in III-V GAA transistor electrostatics. The quantum mechanical effects arising because of the highly confined GAA geometry is also included. The proposed model is compared with the data obtained from 2D Poisson-Schrodinger solver for different III-V channel materials and dimensions and found to be in excellent agreement.

International audience; Noncovalent interactions play a key role in functional materials. Metal–organofluorine interactions are of special interest because they directly affect the structure and reactivity of hybrid fluorinated materials. In-depth understanding and modulating of these interactions would enable the rational design of functional materials from fundamental chemical principles. In this work, we propose a computational approach that enables a comprehensive and quantitative characterization of noncovalent interactions (NCIs) in hybrid fluorinated crystals. Our approach couples dispersion-corrected density functional theory to NCI analysis. Additionally, we determine electron densities at bond critical points and identify electrostatic interactions using a simple electrostatic model. The versatility of this approach to probe a wide range of NCIs is demonstrated for a series of four bimetallic fluorinated crystals incorporating alkali–manganese(II) pairs and trifluoroacetato ligands. Noncovalent interactions in these hybrid crystals include metal–oxygen, metal–fluorine, hydrogen bonds, and van der Waals forces. Using K2Mn2(tfa)6(tfaH)2·H2O as an example, we demonstrate that its two-dimensional layered structure stems from a unique balance between these four NCIs. The computational approach presented herein should have general applicability to the quantitative study of NCIs in hybrid crystals, thereby serving as a guide for crystal engineering of novel hybrid materials.

Intermolecular interactions, in regard to which people tend to emphasise the noncovalent van der Waals (vdW) forces when conducting investigations throughout chemistry, can influence the structure, stability and function of molecules and materials. Despite the ubiquitous nature of vdW interactions, a simplified electrostatic model has been popularly adopted to explain common intermolecular interactions, especially those existing in π-involved systems. However, this classical model has come under fire in revealing specific issues such as substituent effects, due to its roughness; and it has been followed in past decades by sundry explanations which sometimes bring in nebulous descriptions. In this account, we try to summarise and present a unified model for describing and analysing the binding mechanism of such systems from the viewpoint of energy decomposition. We also emphasise a commonly ignored factor – orbital interaction, pointing out that the noncovalent intermolecular orbital interactions a...

We use a two-dimensional electrostatic model to explore Van der Pauw (VdP) method on samples with holes. Results of electrostatic simulation based on the model are consistent with the experimental results. The quantitative investigation of VdP method on sample with one hole indicate the value of f, i.e. the right hand side of VdP equation, is depend on the position of contacts and the size of holes. We also quantitatively explore the VdP method used on sample with two holes in special position of contacts and sample geometry. According to the guidance of simulation and experiment, we find proper position of contacts, which make VdP equation f = 1 valid again on sample with holes. It is helpful for the optimization of VdP method and measuring the resistivity of sample with holes.

We extend the linearized Poisson-Boltzmann (LPB) continuum electrostatic model for molecular solvation to address charge-hydration asymmetry. Our new solvation-layer interface condition (SLIC)/LPB corrects for first-shell response by perturbing the traditional continuum-theory interface conditions at the protein-solvent and the Stern-layer interfaces. We also present a GPU-accelerated treecode implementation capable of simulating large proteins, and our results demonstrate that the new model exhibits significant accuracy improvements over traditional LPB models, while reducing the number of fitting parameters from dozens (atomic radii) to just five parameters, which have physical meanings related to first-shell water behavior at an uncharged interface. In particular, atom radii in the SLIC model are not optimized but uniformly scaled from their Lennard-Jones radii. Compared to explicit-solvent free-energy calculations of individual atoms in small molecules, SLIC/LPB is significantly more accurate than standard parametrizations (RMS error 0.55 kcal/mol for SLIC, compared to RMS error of 3.05 kcal/mol for standard LPB). On parametrizing the electrostatic model with a simple nonpolar component for total molecular solvation free energies, our model predicts octanol/water transfer free energies with an RMS error 1.07 kcal/mol. A more detailed assessment illustrates that standard continuum electrostatic models reproduce total charging free energies via a compensation of significant errors in atomic self-energies; this finding offers a window into improving the accuracy of Generalized-Born theories and other coarse-grained models. Most remarkably, the SLIC model also reproduces positive charging free energies for atoms in hydrophobic groups, whereas standard PB models are unable to generate positive charging free energies regardless of the parametrized radii. The GPU-accelerated solver is freely available online, as is a MATLAB implementation.

Lone pair-π interactions are now recognized as a supramolecular bond whose existence in biological systems is documented by a growing number of examples. They are commonly attributed to electrostatic forces. This review attempts to highlight some recent discoveries evidencing the important role which lone pair-π interactions, and anion-π interactions in particular, play in stabilizing the structure and affecting the function of biomolecules. Special attention is paid to studies exploring the physical origin of these at first glance counterintuitive interactions between a lone pair of electrons of one residue and the π-cloud of another. Recent theoretical work went beyond the popular electrostatic model and inquired the extent to which orbital interactions have to be taken into account. In at least one biologically relevant case-that of anion-flavin interactions-a substantial charge-transfer component has been shown to operate.

Our manuscript describes the implementation and performance of Ewald-based approaches for the calculation of Gaussian integrals in the pmemd.gem code as released in the AMBER18 suite, as well as a new parametrization of our density-based Gaussian Electrostatic Model* (GEM*) force field using CCSD(T)/CBS//SAPT2+3/aug-cc-pVTZ data for the fit.

The QM/MM method has become a useful tool to investigate various properties of complex systems. We previously introduced the layered interacting chemical models (LICHEM) package to enable QM/MM simulations with advanced potentials by combining various (unmodified) QM and MM codes ( J. Comp. Chem., 2016, 37, 1019). LICHEM provides several capabilities such as the ability to use polarizable force fields, such as AMOEBA, for the MM environment. Here, we describe an updated version of LICHEM (v1.1), which includes several new functionalities including a new method to account for long-range electrostatic effects in QM/MM (QM/MM-LREC), a new implementation for QM/MM with the Gaussian electrostatic model (GEM), and new capabilities for path optimizations using the quadratic string model (QSM) coupled with restrained MM environment optimization.

In this paper, we propose a structure for graphene spaser and develop an electrostatic model for quantizing plasmonic modes. Using this model, one can analyze any spaser consisting of graphene in the electrostatic regime. The proposed structure is investigated analytically and the spasing condition is derived. We show that spasing can occur in some frequencies where the Quality factor of plasmonic modes is higher than some special minimum value. Finally, an algorithmic design procedure is proposed, by which one can design the structure for a given frequency. As an example, a spaser with plasmon energy of 0.1 eV is designed.

The dynamics of the electrochemical double layer in ionic liquids can be experimentally probed by a number of experimental techniques. Earlier, we reported on the results of an X-ray photoelectron spectroscopic investigation under applied square-wave bias of two distinct frequencies. Our studies had revealed counterintuitive properties involving the physical and temporal progression of the effect of the electrochemical double layer that could not be modeled with conventional equivalent-circuit approaches. Herein, we present a new coarse-grained modeling methodology that accounts for particle diffusion, migration, and ion-association equilibrium. Our model is computationally efficient enough to be used to predict and match the results at extended time scales and distances of the experiment. Modeling efforts predict that a crucial component of the behavior is controlled by the ion-pairing equilibrium, an often overlooked aspect of ionic liquids.

A periodic homogenization model of the electrostatic equation is constructed for a comb drive with a large number of fingers and whose mode of operation is in-plane and longitudinal. The model is obtained in the case where the distance between the rotor and the stator is of an order $\varepsilon ^{\alpha }$, $\alpha \geq 2$, where $\varepsilon$ denotes the period of distribution of the fingers. The model derivation uses the two-scale convergence technique.\ Strong convergences are also established. This allows us to find, after a proper scaling, the limit of the electrostatic force applied to the rotor in the longitudinal direction.

The optical properties of thin gold films with thickness varying from 2.5 nm to 30 nm are investigated. Due to the quantum size effect, the optical constants of the thin gold film deviate from the Drude model for bulk material as film thickness decreases, especially around 2.5 nm, where the electron energy level becomes discrete. A theory based on the self-consistent solution of the Schrödinger equation and the Poisson equation is proposed and its predictions agree well with experimental results.

A mechanism-oriented model is proposed here as a speculative but robust attempt to understand whether there might be any increased risk of electrostatically induced contamination, with relevant consequences from the epidemiological viewpoint. This could also be the case for the COVID-19 spreading because an amount of micro-sized droplet nuclei, often carrying net electric charge, are expected to be electro-dynamically involved in a physical process originated by the natural and unperceivable static electrification of human beings. The effects of the triboelectric charging have long been successfully tested because the phenomenon under examination is also implied in the genesis of the electrostatic discharge (ESD), a demanding key objective in the special context of electromagnetic compatibility (EMC). Therefore, the ultimate purpose of this technical paper is to provide valuable insights into infection control, building on what is already being done for maintaining static-safe environments. The stature of the applied model can be further appreciated because some currently observed climate-dependent and sex-linked different vulnerabilities to COVID-19 are critically examined by unique sound arguments. These ultimately focus attention on ambient relative humidity and worn shoes, the latter differing for typology, size and material, for their integrated control of the inadvertent human aptitude to buildup tribocharges. These would appear as a dreadful prerequisite for charge bearing droplets in the airborne state to be efficiently attracted/repelled according to the described electrostatic mechanism.

The mechanism of disjoining pressure controlling the swelling and water absorption of gel structures for soil conditioning is discussed. Its quantitative description is based on the fundamental ion-electrostatic model of soil water sorption isotherms containing the phase variable of gel swelling degree and specific surface area index of the soil substrate. The model adequately reproduces the s-shape of soil water sorption isotherms in a wide range of water activity from 0.05 to 0.999 depending on the working doses of hydrogels in the range of 0.05-0.3 mass%. Reinforcement of gel structures with amphiphilic dispersed particles increases the effect of disjoining pressure and water absorption.

© 2017, Indian Academy of Sciences. The environment of a solute molecule in an ionic liquid is likely to have large fluctuating electrostatic fields, and so the electrostatic properties of such a solute including its charge distribution and its polarizability may make a difference to both its static and dynamic properties. We have developed a new model for the static electrostatic distribution in the CS 2 molecule with 7 charged sites and anisotropic polarizability on the carbon site and isotropic polarizability on the sulfurs. We have investigated static and dynamic properties of the neat liquid and solutions of CS 2 in an ionic liquid, [dmim][NTf 2 ]. [Figure not available: see fulltext.] .

Single-molecule transistor experiments offer the ultimate probe of a molecule’s redox chemistry. With a combined quantum mechanics/molecular mechanics computational method and an electrostatic model of the junction, the Coulomb blockade diamonds can be reconstructed. We present computations on 1,4-diamino-benzene (OPVN) in a molecular transport junction, and focus on the chemical features such as electrostatic stabilization, importance of binding group, and binding distance. Thus the explicit construction of the Coulomb blockade diamonds offers a method for direct comparison between different molecular junctions and thus also with experiments.

The electrodynamic screen, or EDS, has shown promising results in mitigation of dust accumulation losses in solar energy harvesting systems. In this paper, the electric field distributions in two EDS configurations have been thoroughly investigated. The analytical solutions for the electric potential and electric field distribution in the first EDS configuration have been provided and corroborated numerically using finite element analysis (FEA) software. The electrostatic model of second EDS configuration has been developed in the FEA software and its electric field distribution has been analyzed numerically. A comparison has been made between the two configurations regarding dust removal efficiency.

This paper describes four programs for Windows, designed to obtain the thermodynamic properties of aqueous species from experimental data and reporting them in the Unitherm database format (HCh software package). Programs OptimA and OptimS allow users to derive the standard Gibbs free energies of aqueous species from the results of chemical experiments (e.g., potentiometry or solubility) and from ultraviolet–visible (UV–Vis) absorption spectra, respectively; programs OptimB and OptimC enable optimization of the parameters of the revised Helgeson–Kirkham–Flowers equation of state and the modified Ryzhenko–Bryzgalin electrostatic model, respectively, for an aqueous species from its standard Gibbs free energy or stability constant as a function of temperature and pressure.

A new method of lightning protection has been developed in which the lightning leader is attracted to ground objects as a result of the electrostatic interaction between the leader charge and induced charges on ground objects.

In this paper we apply a detailed electrostatic model of an aircraft to be used in an experimentally validated, new electric-charge-based circuit model for studying aircraft-lightning electrodynamics. The model is used to evaluate the electrodynamics of an aircraft under a thundercloud. As commercial and military aircraft continue to be subject to direct lightning flashes, we have previously developed a dipole model to characterize electrical currents and electric potential fluctuations on an aircraft for alternative design strategies to minimizing the severity of lightning-aircraft dynamics. With the increased severity of thunderstorms due to global warming, the need to predict and quantify electrical characteristics of the lightning-aircraft electrodynamics is greater, but they are normally not measurable. That dipole model is used here in a new a simple matrix formulation and applied to low-flying aircraft to compute the lightning channel voltages and currents after the aircraft is struck by li...

An electrostatic model based on the effective-point-charge approach is proposed to predict the quantized axis of rare-earth ions, even in low symmetry. The effective charges of coordination atoms create an electrostatic potential interacting with the aspherical 4f electron cloud of the rare-earth ion. The quantized axis can be determined by minimizing the electrostatic energy. The effective charge displacements must be considered according to the nature of the coordination bond, which can be calculated by the atomic orbital wavefunctions for the σ- and π-bonds, respectively. Using our model, the experimentally determined magnetic easy axis of some complexes with Tb3+, Dy3+ and Er3+ can be very well reproduced.

Spectroscopic data on the 3d 1 transition metal activators Ti 3+ , V 4+ and Cr 5+ in compounds were collected and analysed in order to obtain an overview on the size of the 3d 1 level crystal field splitting. Results are compared with data on the 5d 1 activators Ce 3+ and Eu 2+ . Trends earlier observed for the 5d 1 lanthanides also apply to the 3d 1 transition metals. They will be interpreted with the point charge electrostatic model and a modified version thereof.

Recent experimental data on the hydrolysis of U(VI), Pu(VI), Np(V), and Th(IV) at variable temperatures are summarized in this review. Data indicate that the hydrolysis reactions of U(VI), Pu(VI), Np(V), and Th(IV) are all enhanced when temperature is increased from 283 to 358 K. In general, the tendency of actinide elements in different oxidation states toward hydrolysis follows the order: An(IV) > An(VI) > An(V), which can be well described by the electrostatic model. The enhancement of hydrolysis at higher temperatures can be attributed to the increase of ionization of water with the increase of temperature. A few theoretical thermodynamic approaches for predicting the effect of temperature, including the constant enthalpy approach, the constant heat capacity approach, the DQUANT equation, and the Ryzhenko-Bryzgalin model, are tested with the experimental data.

© 2017, Indian Academy of Sciences. The environment of a solute molecule in an ionic liquid is likely to have large fluctuating electrostatic fields, and so the electrostatic properties of such a solute including its charge distribution and its polarizability may make a difference to both its static and dynamic properties. We have developed a new model for the static electrostatic distribution in the CS 2 molecule with 7 charged sites and anisotropic polarizability on the carbon site and isotropic polarizability on the sulfurs. We have investigated static and dynamic properties of the neat liquid and solutions of CS 2 in an ionic liquid, [dmim][NTf 2 ]. [Figure not available: see fulltext.] .

Background. The selectivity of biological cation channels is defined by a short, narrow selectivity filter, having a negative net fixed charge Qf . Voltage gated bacterial channels (NaChBac and some others) are frequently used in biophysics as simplified models of mammalian calcium and sodium channels. We report an experimental, analytic and numerical study of the effects of Qf and bulk ionic concentrations of Ca2+ and Na+ on conduction and selectivity of NaChBac channels, wild type and Qf -varied mutants. Methods. Site-directed mutagenesis and voltage clamp recordings were used to investigate the Na+ /Ca2+ selectivity, divalent blockade and anomalous mole fraction effect (AMFE) for different NaChBac wild type/mutants channels and the properties dependence on Qf . Experimental results were compared with Brownian dynamics simulations and with analytic predictions of the ionic Coulomb blockade (ICB) model, which was extended to encompass bulk concentration effects. Results. It was shown that changing of Qf from –4e (for LESWAS wild type) to –8e (for LEDWAS mutant) leads to strong divalent blockade of the Na+ current by micromolar amounts of Ca2+ ions, similar to the effects seen in mammalian calcium channels. The BD simulations revealed a concentration-related logarithmic shift of the conduction bands. These results were shown to be consistent with ICB model predictions. Conclusions. The extended ICB model explains the experimental (divalent blockade and AMFE) and simulated (multi-ion bands and their concentration-related shifts) selectivity phenomena of NaChBac channel and its charge-varied mutants. These results extend the understanding of ion channel selectivity and may also be applicable to biomimetic nanopores with charged walls.

In this paper, it is shown that Stieltjes electrostatic model and quantum Hamilton Jacobi formalism are analogous to each other. This analogy allows the bound state problem to mimic as n unit moving imaginary charges $i\hbar $ , which are placed in between the two fixed imaginary charges arising due to the classical turning points of the potential. The interaction potential between n unit moving imaginary charges $i\hbar $ is given by the logarithm of the wave function. For an exactly solvable potential, this system attains stable equilibrium position at the zeros of the orthogonal polynomials depending upon the interval of the classical turning points.

Current cutoff in lightning channels, which takes place in the development of both intracloud and cloud-to-ground flashes, is still poorly understood. A new evaluation of the conditions leading to current cutoff, and also of the two existing hypotheses of the cutoff mechanism, is the main objective of the paper. We reviewed the literature with results of measurements and modeling of free-burning arcs in a laboratory (the closest analogs of lightning leaders) focusing on the relationship between the internal electric field and current. This relationship governs the leader's behavior in the current cutoff. In our analysis of the mechanisms leading to current cutoff, we identify the two types of current cutoff in lightning channels: the current cutoff in a single, unbranched leader channel, which occurs as the result of reaching the threshold conditions for leader propagation; and the current cutoff in branched leaders, when screening by the leader branches alters the ambient electrical environment, thus diminishing the leader current and causing cutoff at a branching point or at the base of the straight channel that preceded branching. We advance the electrostatic model of the screening effect of branching on current cutoff, introduced by Mazur and Ruhnke (1993), and we provide the evidence of this mechanism from lightning observations. We also critically evaluate the concept of lightning-channel instability, proposed by Heckman (1992), as a suggested mechanism leading to current cutoff. We show that the fundamentals of this concept and therefore the concept in its entirety are invalid.

It is observed that the electrostatic potential created by the ions of an infinite one-dimensional ionic crystal at the position of a reference ion (for instance, a positive ion) is equivalent to the electrostatic potential created at the same point by a system of two identical uniformly charged straight wires each containing a charge equal to the opposite charge of the reference ion. The distance of the closest point of each wire relative to the reference ion and the length of the wires is equal to the distance separating any two adjacent ions in the regular lattice. Keywords: Ionic crystal, Theory and modeling

We studied the energy levels of Eu3+ at the C2 lattice site of cubic Eu2O3. We identified 29 Lorentzian lines corresponding to 5D0 → 7F0-6 transitions obtained from photoluminescence spectra at 10 K. The levels were analyzed in terms of the 14 crystal field parameters corresponding to the site symmetry. The energy-level fit yielded a final standard deviation of 12.0 cm−1. The resulting crystal field strength parameter is Nv = 3869 ± 185. This value confirms the linear dependence on the host atomic radius predicted by the extended point charge electrostatic model (PCEM). The present study considered the highest number of transitions to date for Eu3+ in Eu2O3 to the authors’ knowledge. Keywords: Europium, Oxides, Luminescence, Crystal field

We performed the first principle calculation to investigate the polarization behavior of BaTiO3(BTO)/PbTiO3(PTO) superlattices with a period-5 superlattice model. The results show that when BTO proportion increases, values of c/a increase and polarizations decrease. In BPT superlattices, polarization in each local layer keeps a constant value, indicating that short-period BPT superlattices can be approximately considered as a single ferroelectric material. Moreover, from analysis of the electrostatic model, we know the directions of internal electric fields in BTO and PTO layers are opposite. Internal electric field in PTO layer leads to polarization loss in this layer, but polarization in BTO layer is enhanced by internal electric field.

International audience; In the present work, we investigate different possibilities for the nonempirical localization of nonatomic centers within the context of the design of new generation polarizable force fields. To do so, the positions of electron localization function (ELF) and electron pair localization function (EPLF) attractors and of Boys orbital centroids are determined for a set of thirteen saturated and conjugated biologically relevant molecules. We consider the similarities and differences in the representations of localized lone pairs and π delocalized systems by these approaches, as well as the effects of the basis sets and of the level of quantum chemistry (QC). All three methods give consistent results upon dealing with the localized lone pairs. Concerning aromatic systems, whereas ELF and EPLF methods give mutually consistent results, the Boys scheme breaks the symmetry by alternating the electron distributions along the C-C bonds providing a different distribution of off-centered points. We then investigate the influence of lone pair localization in the water model of the Gaussian electrostatic model/sum of interactions between fragments ab initio computed (GEM/SIBFA) polarizable force field, which embodies an explicit representation of the lone pairs. This is done, in a series of mono- and poly-hydrated Zn(II) complexes, by comparing the overlap-dependent repulsion (Erep) and charge-transfer (Ect) contributions to their QC counterparts. We resort to either the current GEM/SIBFA water lone pair internal coordinates or to the ones derived from the previously mentioned localization procedures. The latter enables closer reproductions by Erep and Ect (GEM/SIBFA) of their exchange repulsion (Eexch) and Ect QC counterparts. The present preliminary results show that QC localization procedures can be used to derive accurate, nonempirical positions for off-centered points intervening in the formulation of the overlap-dependent contributions of next-generation polarizable force fields.; Dans la présente étude, nous examinons les différentes localisations non empiriques possibles des centres non atomiques dans le contexte de la conception de champs de forces polarisables de nouvelle génération. À cette fin, nous déterminons les positions des attracteurs ELF (Electron Localization Function, ou fonction de localisation électronique) et des attracteurs EPLF (Electron Pair Localization Function, ou fonction de localisation de paire d'électrons), ainsi que des centroïdes des orbitales de Boys pour un jeu de treize molécules saturées et conjuguées ayant une importance biologique. Nous considérons les similarités et les différences entre les représentations des paires libres et des systèmes délocalisés obtenues par ces approches, ainsi que les effets des bases et du niveau de chimie quantique (QC). Les trois méthodes donnent des résultats cohérents lorsque l'on s'intéresse aux paires libres localisées. En ce qui concerne les systèmes aromatiques, alors que les méthodes ELF et EPLF donnent des résultats mutuellement cohérents, la méthode de Boys rompt la symétrie en alternant la répartition des électrons le long des liaisons CC , ce qui donne une distribution différente des points excentrés. Nous étudions ensuite l'influence de la localisation des paires libres dans le modèle de l'eau dans le champ de forces polarisable GEM/SIBFA, qui figure une représentation explicite des paires libres. Sur une série de complexes de Zn(II) mono et polyhydratés, nous comparons les contributions des énergies de répulsion (E rép) et de transfert de charge (E tc) dépendantes de la superposition à leurs équiva-lents dans les calculs de chimie quantique (QC). Nous utilisons les coordonnées internes des paires libres de l'eau de la méthode GEM/SIBFA courante ou celles dérivées des procédures de localisation susmentionnées. Les secondes permettent que E rép and E tc (GEM/SIBFA) donnent des reproductions plus proches de leurs équivalents QC E éch et E tc. Les résultats préliminaires présentés ici montrent que des procédures de localisation basées sur la chimie quantique peuvent être utilisées pour calculer les positions exactes, non empiriques, pour les points excentrés intervenant dans la formulation des contributions dépendantes du recou-vrement des champs de forces polarisables de nouvelle génération. [Traduit par la Rédaction]

Polarizable continuum solvation models are nowadays the most popular approach to describe solvent effects in the context of quantum mechanical calculations. Unexpectedly, despite their widespread use in all branches of quantum chemistry and beyond, important aspects of both their theoretical formulation and numerical implementation are still not completely understood. In particular, in this perspective we focus on the numerical issues of their implementation when applied to large systems and on the theoretical framework needed to treat time dependent problems and excited states or to deal with electronic correlation. Possible extensions beyond a purely electrostatic model and generalizations to environments beyond common solvents are also critically presented and discussed. Finally, some possible new theoretical approaches and numerical strategies are suggested to overcome the obstacles which still prevent a full exploitation of these models.

A simple electrostatic model explains the enhanced stereoselectivity of N-oxide catalyzed allylations compared to propargylations, which in turn explicates the dearth of stereoselective N-oxide propargylation catalysts. These results suggest that N-oxide catalysts that are effective for both allylations and propargylations can be designed by targeting inherently stereoselective ligand configurations and through the manipulation of distortion effects in the operative transition states.

One of the most popular and simple models for the calculation of pK(a) s from a protein structure is the semi-macroscopic electrostatic model MEAD. This model requires empirical parameters for each residue to calculate pK(a) s. Analysis of current, widely used empirical parameters for cysteine residues showed that they did not reproduce expected cysteine pK(a) s; thus, we set out to identify parameters consistent with the CHARMM27 force field that capture both the behavior of typical cysteines in proteins and the behavior of cysteines which have perturbed pK(a) s. The new parameters were validated in three ways: (1) calculation across a large set of typical cysteines in proteins (where the calculations are expected to reproduce expected ensemble behavior); (2) calculation across a set of perturbed cysteines in proteins (where the calculations are expected to reproduce the shifted ensemble behavior); and (3) comparison to experimentally determined pK(a) values (where the calculation should reproduce the pK(a) within experimental error). Both the general behavior of cysteines in proteins and the perturbed pK(a) in some proteins can be predicted reasonably well using the newly determined empirical parameters within the MEAD model for protein electrostatics. This study provides the first general analysis of the electrostatics of cysteines in proteins, with specific attention paid to capturing both the behavior of typical cysteines in a protein and the behavior of cysteines whose pK(a) should be shifted, and validation of force field parameters for cysteine residues.

We present the inclusion of distributed multipoles obtained from the Gaussian Electrostatic Model (GEM) into the AMOEBA force field. As a proof of principle, we have reparametrized water and alanine di-peptide. The GEM distributed multipoles (GEM-DM) have been obtained at the same levels of theory as those used for the original AMOEBA parametrization. The use of GEM allows the derivation of the distributed multipoles from the analytical fit to the molecular density or the numerical fit to the molecular electrostatic potential (mESP). In addition, GEM-DM are intrinsically finite of the highest order of the auxiliary basis used for the GEM fit. We also present the fitting of multipoles for the di-methyl imidazolium/chloride (DMIM(+)-Cl(-)) ionic liquid pair. Results for intermolecular Coulomb for all test systems show very good agreement. MD simulations for a reparametrized AMOEBA water model with GEM-DM provide results on par with the original AMOEBA force field for a series of bulk properties including liquid density and enthalpy of vaporization. A package for the calculation of GEM Hermite coefficients and derived distributed multipoles using the numerical procedure is also presented and released under the GNU public license.

In many cases the stability of a protein has to be increased to permit its biotechnological use. Rational methods of protein stabilization based on optimizing electrostatic interactions have provided some fine successful predictions. However, the precise calculation of stabilization energies remains challenging, one reason being that the electrostatic effects on the unfolded state are often neglected. We have explored here the feasibility of incorporating Poisson-Boltzmann model electrostatic calculations performed on representations of the unfolded state as large ensembles of geometrically optimized conformations calculated using the ProtSA server. Using a data set of 80 electrostatic mutations experimentally tested in two-state proteins, the predictive performance of several such models has been compared to that of a simple one that considers an unfolded structure of non-interacting residues. The unfolded ensemble models, while showing correlation between the predicted stabilization values and the experimental ones, are worse than the simple model, suggesting that the ensembles do not capture well the energetics of the unfolded state. A more attainable goal is classifying potential mutations as either stabilizing or non-stabilizing, rather than accurately calculating their stabilization energies. To implement a fast classification method that can assist in selecting stabilizing mutations, we have used a much simpler electrostatic model based only on the native structure and have determined its precision using different stabilizing energy thresholds. The binary classifier developed finds 7 true stabilizing mutants out of every 10 proposed candidates and can be used as a robust tool to propose stabilizing mutations., We acknowledge financial support from grants BFU2013-47064-P from MINECO (Spain) and Protein Targets group from Diputación General de Aragón, (Spain). We thank J. L. Alonso and N. Cremades for many clarifying discussions.

This work builds on our recently published anisotropic potential for polycyclic aromatic hydrocarbons (PAH) [T.S. Totton, A.J. Misquitta, M. Kraft, J. Chem. Theory Comput. 6 (2010) 683] by developing a new transferable electrostatic model for PAH molecules. Using this model, the atomic charge parameters used in the PAH anisotropic potential may be rapidly calculated from a set of predefined parameters rather than from molecule-specific ab initio calculations. The importance of the out-of-the-plane quadrupolar moments is highlighted and they are used as the basis for an accurate and transferable electrostatic model for PAHs. This model exhibits an r.m.s. deviation of 1.7 kJ mol - 1 - an order of magnitude less than previous models.

Three-dimensional quantitative structure activity relationship (3D-QSAR) studies were performed on Frusectose-1, 6-bisphosphatase (FBPase) inhibitors, based on molecular docking obtained by using GOLD and comparative molecular field analysis (CoMFA). Three random splits into training and test sets had been performed and the high leave-one-out (LOO) cross-validated correlation coefficients q(2) of 0.781, 0.725 and 0.801, respectively, revealed that the models are useful tools for the prediction of test sets as well as newly designed structures against FBPase activity. The superimposed CoMFA models on the receptor site of FBPase are guiding the design of potential inhibitory structures directed against FBPase activity. (C) 2011 Elsevier Masson SAS. All rights reserved.