Solvation Monte Carlo calculations

Obviously this picture might be supported and supplemented by according data from different experimental investigations, or it might be modified to fit these data. Interactions within the basic hydrated structures, as well as their energetics, are obtainable from gas-phase solvation experiments or from accurate MO calculations. For the simulation of real solutions, dynamic calculations will be inevitable. There is, however, a demand for acceptable effective potentials to be used in molecular dynamics, or in Monte Carlo calculations. 

Ab initio and Monte-Carlo calculations. Attempts have appeared in pulse radiolysis to describe the dynamics of free electron production, recombination and solvation on a microscopic scale [31-34]. This requires the knowledge of a number of physical parameters solvated electron and free ion yields, electron and hole mobilities, slowing-down cross-sections, localization and solvation times, etc. The movement and fate of each reactant is examined step by step in a probabilistic way and final results are obtained by averaging a number of calculated individual scenarios. 

Monte Carlo calculations for structure 1 and 2, in order to obtain the two free energy curves. MM interaction potentials of the type shown in eq.(119) are used here. The solute parameters (atomic charges and van der Waals coefficients) are drawn from SCF calculation on the trimer 1 and 2, as well as on other small solvation clusters ... 

Some of the statistical mechanical developments which depend on the power of the computer to carry out the complex calculations are outlined in Sections 10.17 to 10.22 below. Studies on solvation of ions using, in particular, Monte Carlo calculations are given in Chapter 13 on solvation. 

The translational motion of the Li ion in water, for instance, speeds up the solvation by ca. 20% by using of the density functional theory (DFT) with some approximation. The computational progress toward ionic solvation has been arriving to discuss the more comphcate area such as heat capacity changes (by a Monte Carlo calculation), surface effects on aqueous ionic solvation (by a Molecular Dynamics). ... 

Madura and Jorgensen [96] carried out sophisticated ab initio and Monte Carlo calculations for the nucleophilic addition reaction HO -l-HCHO in aqueous solution. In this solution, the free energy gradually grows with the diminishing distance between the reactants from infinity to 3.0 A. In this region, an increase in the solvation energy is offset by the increase in the ion-dipole... 

Specific solute-solvent interactions involving the first solvation shell only can be treated in detail by discrete solvent models. The various approaches like point charge models, siipennoleciilar calculations, quantum theories of reactions in solution, and their implementations in Monte Carlo methods and molecular dynamics simulations like the Car-Parrinello method are discussed elsewhere in this encyclopedia. Here only some points will be briefly mentioned that seem of relevance for later sections. 

Monte Carlo simulations provide an alternate approach to the generation of stable conformations. As with HyperChem s other simulation methods, the effects of temperature changes and solvation are easily incorporated into the calculations. 

Ab initio molecular orbital calculations are being used to study the reactions of anionic nucleophiles with carbonyl compounds in the gas phase. A rich variety of energy surfaces is found as shown here for reactions of hydroxide ion with methyl formate and formaldehyde, chloride ion with formyl and acetyl chloride, and fluoride ion with formyl fluoride. Extension of these investigations to determine the influence of solvation on the energy profiles is also underway the statistical mechanics approach is outlined and illustrated by results from Monte Carlo simulations for the addition of hydroxide ion to formaldehyde in water. 

MD simulations in expHcit solvents are stiU beyond the scope of the current computational power for screening of a large number of molecules. However, mining powerful quantum chemical parameters to predict log P via this approach remains a challenging task. QikProp [42] is based on a study [3] which used Monte Carlo simulations to calculate 11 parameters, including solute-solvent energies, solute dipole moment, number of solute-solvent interactions at different cutoff values, number of H-bond donors and acceptors (HBDN and HBAQ and some of their variations. These parameters made it possible to estimate a number of free energies of solvation of chemicals in hexadecane, octanol, water as well as octanol-water distribution coefficients. The equation calculated for the octanol-water coefficient is ... 

Tunon et al.194 studied the water molecule in liquid water. The sample of conformations by the microscopic environment (water in this case) was obtained using Monte Carlo technique. The energy was calculated as in the approach of Stanton et al.189 i.e., using Eqs. 4.25 and 4.26. The solvent induced increase of the dipole moment amounted to 0.61 Debye in line with the results by Wei and Salahub and close to the experimental value of 0.75 Debye. The solvation enthalpy amounted —12.6 kcal/mol, while the value calculated by Salahub and Wei and the experimental ones were —10.4 kcal/mol and —9.9 kcal/mol, respectively. 

Marchi, M. Sprik, M. Klein, M. L., Calculation of the free energy of electron solvation in liquid ammonia using a path integral quantum Monte Carlo simulation, J. Phys. Chem. 1988, 92, 3625-3629... 

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