Determining the electronic structure and chemical potentials of molecules in solution

A simulation scheme is proposed for determining the excess chemical potential of a substance in solution. First, a Monte Carlo simulation is performed with classical models for solute and solvent molecules. A representative sample of these configurations is then used in a hybrid quantum/classical (QM/MM) calculation, where the solute is treated quantum-mechanically, and the average electronic structure is used to construct an improved classical model. This procedure is iterated to self-consistency in the classical model, which in practice is attained in one or two steps, depending on the quality of the initial guess. The excess free energy of the molecule within the QM/MM approach is determined relative to the classical model using thermodynamic perturbation theory with a cumulant expansion. The procedure provides a method of constructing classical point charge models appropriate for the solution and gives a measure of the importance of solvent fluctuations.

Concertedness and solvent effects in multiple proton transfer reactions: The formic acid dimer in solution

The issue of multiple proton transfer (PT) reactions in solution is addressed by performing molecular dynamics simulations for a formic acid dimer embedded in a water cluster. The reactant species is treated quantum mechanically, within a density functional approach, while the solvent is represented by a classical model. By constraining different distances within the dimer we analyze the PT process in a variety of situations representative of more complex environments. Free energy profiles are presented, and analyzed in terms of typical solvated configurations extracted from the simulations. A decrease in the PT barrier height upon solvation is rationalized in terms of a transition state which is more polarized than the stable states. The dynamics of the double PT process is studied in a low-barrier case and correlated with solvent polarization fluctuations. Cooperative effects in the motion of the two protons are observed in two different situations: when the solvent polarization does not favor the transfer of one of the two protons and when the motion of the two protons is not synchronized. This body of observations is correlated with local structural and dynamical properties of the solvent in the vicinity of the reactant. (C) 2000 American Institute of Physics. [S0021-9606(00)51121-0].

Competitive adsorption of reactive dyes from solution: equilibrium isotherm studies in single and multisolute systems

Experimental data of the adsorption of reactive dyestuffs onto Filtrasorb 400 activated carbon (FS400) were determined in an equilibrium isotherm study. As most industrial wastewater contains more than one pollutant, an investigation into the effect of multisolute systems (using the unhydrolysed form of the reactive dyes) on the adsorption capacity was undertaken. Equilibrium isotherm models were employed to describe the adsorption capacities of single, binary and ternary dye solutions. The results of these analyses showed that adsorption of reactive dyes from single and multisolute systems can be successfully described by Langmuir, and Redlich–Peterson equilibrium isotherm models. Experimental data indicated that competitive adsorption for active sites on the carbon surface results in a reduction in the overall uptake capacity of the reactive dyes investigated.

Packet classification by multi-level cutting of the classification space: An algorithmic-architectural solution for IP packet classification in next generation networks

Traditionally, the Internet provides only a “best-effort” service, treating all packets going to the same destination equally. However, providing differentiated services for different users based on their quality requirements is increasingly becoming a demanding issue. For this, routers need to have the capability to distinguish and isolate traffic belonging to different flows. This ability to determine the flow each packet belongs to is called packet classification. Technology vendors are reluctant to support algorithmic solutions for classification due to their non-deterministic performance. Although CAMs are favoured by technology vendors due to their deterministic high lookup rates, they suffer from the problems of high power dissipation and high silicon cost. This paper provides a new algorithmic-architectural solution for packet classification that mixes CAMs with algorithms based on multi-level cutting the classification space into smaller spaces. The provided solution utilizes the geometrical distribution of rules in the classification space. It provides the deterministic performance of CAMs, support for dynamic updates, and added flexibility for system designers.