Search results for "CONST"
showing 10 items of 7706 documents
Exploiting seeding of random number generators for efficient domain decomposition parallelization of dissipative particle dynamics
2013
Abstract Dissipative particle dynamics (DPD) is a new promising method commonly used in coarse-grained simulations of soft matter and biomolecular systems at constant temperature. The DPD thermostat involves the evaluation of stochastic or random forces between pairs of neighboring particles in every time step. In a parallel computing environment, the transfer of these forces from node to node can be very time consuming. In this paper we describe the implementation of a seeded random number generator with three input seeds at each step which enables the complete generation of the pairwise stochastic forces in parallel DPD simulations with minimal communication between nodes.
Mapping the Teaching of History of Chemistry in Europe
2016
Recent developments in the field of history of chemistry have introduced new topics, challenges, and connections to a broad range of scientific, political, cultural, economic, and environmental issues. New audiences for the history of chemistry have emerged along with new topics, protagonists, spaces, and historical narratives. This paper summarizes the main results of a recent survey to map the current situation of the teaching of history of chemistry in Europe. We review how and where history of chemistry is taught in Europe, considering not only graduate students in science programs, but also other audiences such as science teachers, and the general public. This paper also provides updat…
Revised Atomistic Models of the Crystal Structure of C–S–H with high C/S Ratio
2016
Abstract The atomic structure of calcium-silicate-hydrate (C1.67–S–H x ) has been studied. Atomistic C–S–H models suggested in our previous study have been revised in order to perform a direct comparison of energetic stability of the different structures. An extensive set of periodic structures of C–S–H with variation of water content was created, and then optimized using molecular dynamics with reactive force field ReaxFF and quantum chemical semiempirical method PM6. All models show organization of water molecules inside the structure of C–S–H. The new geometries of C–S–H, reported in this paper, show lower relative energy with respect to the geometries from the original definition of C–S…
Ab initio modelling of the effects of varying Zr (Ti) concentrations on the atomic and electronic properties of stoichiometric PZT solid solutions
2017
Abstract Lead zirconate titanate Pb(ZrxTi1−x)O3 solid solution is considered as one of the most advanced ferroelectric and piezoelectric materials. Consequent variation of Zr (Ti) concentrations significantly affects the atomic and electronic properties of PZT structures. To perform ab initio modelling of different morphologies for lead zirconate titanate, we are using approach of hybrid density functional B3PW as implemented in CRYSTAL14 computer code. In this study, we are performing large-scale calculations of such PZT parameters as optimized lattice constants, atomic charges and bond populations, as well as band structure (e.g., band gap) and density of states.
Generalized transport coefficients in a gas with large shear rate
1987
We get a solution of the Bhatnagar-Gross-Krook (BGK) model kinetic equation by means of a perturbative expansion of a temperature gradient to study the transport properties in a gas with large shear rate. The irreversible fluxes are evaluated exactly to first order in the expansion for Maxwell molecules. The transport coefficients obtained are highly nonlinear functions of the shear rate. This dependence on shear rate is analysed and compared with previous results for several transport coefficients. Finally, we have found a solution for a simple model of constant collision frequency for which a large shear rate coexists with an arbitrary temperature gradient.
A theoretical and experimental study of the formation mechanism of 4-X-chalcones by the Claisen–Schmidt reaction
2000
Abstract A theoretical and experimental study on the formation of 4-X-chalcones (X=H, Cl, F, OCH3, N(CH3)2) was carried out by the Claisen–Schmidt reaction. The influence of pH and temperature was analyzed. Under the adopted experimental conditions 4-X-chalcones were the only reaction products. In the theoretical study, the thermodynamic magnitudes and the reactivity indexes calculated with the AM1 method were used. The mechanism of Nayak and Rout was analyzed critically and a new mechanism was proposed, characterized by: (1) rapid nucleophilic attack of the catalyst (OH−) on the carbon atom of the methyl group of acetophenone; (2) attacks of the acetophenonate ion on the carbon atom of the…
Photocatalytic degradation of 4-Nitrophenol by g-C3N4-MCy: Mechanism study and kinetic modeling
2021
Abstract A graphitic carbon nitride (g-C3N4-MCy) photocatalyst was prepared by thermal condensation of melamine and cyanuric acid with equal proportions. The photocatalyst was characterized by Fourier transform infrared spectroscopy (FTIR), Specific surface area (SSA), X-ray diffraction (XRD), Field Emission Scanning Electron Microscope (FESEM), Transmission electron microscope (TEM), and DRS–UV–Vis spectroscopic methods. We studied the photo-degradation kinetics of 4-nitrophenol using a g-C3N4-MCy. Four parameters were applied including initial concentration of 4-nitrophenol, amount of catalyst, dissolved oxygen content, and photon flux. The reaction rate constant was shown to be correlate…
Spatially limited diffusion coupled with ohmic potential drop and/or slow interfacial exchange: a new method to determine the diffusion time constant…
2004
Abstract We have analyzed chronoamperometric curves, I ( t ), after small-amplitude potential steps Δ E (PITT technique) for the model of linear diffusion of a species inside an electroactive film, taking into account ohmic effects in the external media (solution and electrode) as well as a finite rate of the interfacial exchange. For its short-time interval, t ≪ τ d ( τ d is the diffusion time constant, corresponding to unlimited diffusion from the interface), three approximate analytical expressions have been proposed. One of these represents an interpolation formula between the value of the current at the start of the diffusion process, I (0)=Δ E / R ext (after the end of the EDL chargin…
A kinetic interpretation of a negative time constant in impedance equivalent circuits for the dissolution/passive transition
2007
Abstract A theoretical impedance function is proposed for the active/passive transition of nickel in an acid sulphate medium. It is considered that passivating species progressively covers the electrode surface. This approximated model predicts the appearance of negative time constants in the impedance spectra when coverage coefficient θ values for passive species are greater than 0.5.
A molecular electron density theory (MEDT) study of the role of halogens (X2= F2, Cl2, Br2and I2) on the aza-Michael-addition reactions
2020
Using the Molecular Electron Density Theory (MEDT), a set of uncatalyzed and X2 molecule-catalyzed (X2 = F2, Cl2, Br2, or I2) aza Michael addition reactions (aza-MARs) between pyrrolidine (PYR) and methyl acrylate (MA) have been studied in the gas phase and in dichloromethane (DCM) medium from the kinetics and molecular mechanism aspects. Analysis of the conceptual DFT reactivity indices indicates that the coordination of the X2 molecules to the oxygen of the MA's carbonyl group increases both the softness and the electrophilic character of MA, thus explaining the catalytic role of the X2 molecules. The nucleophilic attack of the nitrogen of PYR on the MA's β conjugated carbon is the rate-d…