Electrostatic decoupling

A more sophisticated and generalized version of the ideas in electrostatic decoupling has been described.22 24 Multiple X values, (Xj, X2, Xx. ..) are introduced into the potential function, replacing the single X value that has been described, and subject to the boundary conditions... 

Rao and Singh32 calculated relative solvation free energies for normal alkanes, tetra-alkylmethanes, amines and aromatic compounds using AMBER 3.1. Each system was solvated with 216 TIP3P water molecules. The atomic charges were uniformly scaled down by a factor of 0.87 to correct the overestimation of dipole moment by 6-31G basis set. During the perturbation runs, the periodic boundary conditions were applied only for solute-solvent and solvent-solvent interactions with a non-bonded interaction cutoff of 8.5 A. All solute-solute non-bonded interactions were included. Electrostatic decoupling was applied where electrostatic run was completed in 21 windows. Each window included 1 ps of equilibration and 1 ps of data... 

The cell model is a commonly used way of reducing the complicated many-body problem of a polyelectrolyte solution to an effective one-particle theory [24-30]. The idea depicted in Fig. 1 is to partition the solution into subvolumes, each containing only a single macroion together with its counterions. Since each sub-volume is electrically neutral, the electric field will on average vanish on the cell surface. By virtue of this construction different sub-volumes are electrostatically decoupled to a first approximation. Hence, the partition function is factorized and the problem is reduced to a singleparticle problem, namely the treatment of one sub-volume, called cell . Its shape should reflect the symmetry of the polyelectrolyte. Reviews of the basic concepts can be found in [24-26]. 

Blochl PE, Electrostatic Decoupling of Periodic Images of Plane-Wave-Expanded Densities and Derived Point Charges, J Chem Phys, 103, 7422-7428 (1995)... 

A more general interface geometry is shown in Fig. 3C. Physically this corresponds to an infinite array of M/C thin film couples separated by vacuum. A salient point is that the vacuum layers should be thick enough that adjacent M/C slabs do not interact. Interaction is possible in two ways either via electronic wavefunction overlap in the vacuum or via Coulombic multipoles. The former interaction is usually vanishing, if more than 1 10 A of vacuum is present. The latter interaction is rather long ranged, but fortunately methods have been devised to electrostatically decouple the slabs.Of course, it is required that both the metal and ceramic layers are thick enough that the interface and surfaces do not interact. 

From (2.70), it follows that the free energy cannot be divided simply into two terms, associated with the interactions of type a and type b. There are also coupling terms, which would vanish only if fluctuations in AUa and AUb were uncorrelated. One might expect that such a decoupling could be accomplished by carrying out the transformations that involve interactions of type a and type 6 separately. In Sect. 2,8.4, we have already discussed such a case for electrostatic and van der Waals interactions in the context of single-topology alchemical transformations. Even then, however, correlations between these two types of interactions are not... 

Previous studies have shown that there is a correlation between the enthalpy of hydration of alkanes and their accessible surface area [30,31] or related magnitudes. Moreover, relationships between the hydration numbers calculated from discrete simulations for hydrocarbons and both the free energy and enthalpy of hydration of these molecules have also been reported [32] and have been often used to evaluate solvation enthalpies. Analysis of our results, illustrates the existence of a linear relationship between A//n eie and the surface of the van der Waals cavity,. SVw, defined in MST computations for the calculation of the non-electrostatic contributions (Figure 4-1). In contrast, no relationship was found for the electrostatic component of the hydration enthalpy (A//eie data not shown). Clearly, in a first approximation, one can assume that the electrostatic interactions between solute and solvent can be decoupled from the interactions formed between uncharged solutes and solvent molecules. 

In the EH-CSD approach it is not convenient to decouple electrostatic terms into rigid Coulombic and polarization contributions the effective Hamiltonian leads to compute these two terms together. Exchange repulsive terms are hardly computed when the second partner of the interaction is a liquid they may be obtained with delicate simulation procedures, and it is convenient to decouple them into two contributions, namely the work spent to form a cavity of a suitable shape and an additional repulsion contribution. Dispersion contributions may be kept we shall examine this term in more detail later. Charge-transfer contributions are damped in liquids their inclusion could introduce additional problems in the definition of Vjnt via continuous solvent distributions. It is advisable to neglect them, as it is done in the interaction potentials used in simulations with the present approach it is possible to describe the charge transfer effect by enlarging the solute M —> M-Sn. 

Due to the symmetry of the present ion-dipole model, the WOZ equation decouples into electrostatic and nonelectrostatic parts. The nonelectrostatic part has the form of WOZ equation for the two-component dimerizing-nondimerizing mixture [67], Using the Wertheim-Baxter factorization technique, the solution of WOZ equation for the electrostatic part was performed in terms of energy parameters [68, 69],... 

In this section, we will review some of the results obtained for homogeneous fluids. The focus of the section strongly reflects the author s particular interest rather than a complete review of all work done in this area. To a large extent, we will concentrate on aspects that have not been reviewed previously, or on areas that developed since those reviews. The first section deals with the influence of electrostatic interactions on the structure factor, and we stress the decoupling of dipole-dipole interactions from the structure factor, although there is a strong effect on particular g y r) s. In Section V.B we consider the dielectric constant obtained from the CSL equation with particular reference to the influence of shape forces in the dielectric properties. Section V.C considers the application of interaction site theories to calculate thermodynamic properties and fluid phase equilibria. 

This chapter will discuss our investigations into the fundamental non-covalent enzyme-cofactor interactions controlling the redox behavior and physical properties of bio-organic cofactors, as well as studies on supramolecular model systems designed to decouple the effects of the different electrostatic and other interactions. Research towards the design and fabrication of molecular devices based on these same principles will be presented. 

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