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Within the field of computational chemistry, solvent models are a variety of methods to account for the behavior of solvated condensed phases. Solvent models allow simulations and calculations of reactions and processes which take place in solvated phases. These include biological, chemical and environmental processes.〔 Such calculation can lead to predictions and improved understanding of the physical processes occurring. Such models have been extensively tested and reviewed in scientific literature. The various models have their own pros and cons. Implicit models are generally computationally efficient and can provide a reasonable description of the solvent behaviour, but fail to account for the local fluctuations in solvent density around a solute molecule. The density fluctuation behaviour is due to solvent ordering around a solute and is particularly prevalent when one is considering water as the solvent. Explicit models are often less computationally economical, but can provide a physical spatially resolved description of the solvent. However, many of these explicit models are computaionally demanding and can fail to reproduce some experimental results, often due to certain fitting methods and parametrization. Hybrid methodologies are another option. These methods incorporate aspects of implicit and explicit aiming to minimse computational cost whist retaining at least some spatial resolution of the solvent. These methods can require more experience to use them correctly and often contain post calculation correction terms. == Implicit models == (詳細はdielectric constant (ε), this is often supplemented with further parameters, for example solvent surface tension. The dielectric constant is the value responsible for defining the degree of polarizability of the solvent. Generally speaking, for implicit solvents, a calculation proceeds by encapsulating a solute in a tiled cavity (See the figure below). The cavity containing the solute is embedded in homogeneously polarizable continuum describing the solvent. The solute's charge distribution meets the continuous dielectric field at the surface of the cavity and polarizes the surrounding medium, which causes a change in the polarisation on the solute. This defines the reaction potential, a response to the change in polarisation. This recursive reaction potential is then iterated to self-consistency. Continuum models have widespread use, including use in force field methods and quantum chemical situations. In quantum chemistry, where charge distributions come from ''ab initio'' methods (Hartree-Fock (HF), Post-HF and Density Functional Theory (DFT)) the implicit solvent models represent the solvent as a perturbation to the solute Hamiltonian. In general, mathematically, these approaches can be thought of in the following way:〔 Note here that the implicit nature of the solvent is shown mathematically in the equation above, as the equation is only dependent on solute molecule coordinates . The second right hand term is composed of interaction operators. These interaction operators calculate the systems responses as a result of going from a gaseous infinitely separated system to one in a continuum solution. If one is therefore modelling a reaction this process is akin to modelling the reaction in the gas phase and providing a perturbation to the Hamiltonian in this reaction.〔 Top: Four interaction operators generally considered in the continuum solvation models. Bottom: Five contributing free energy terms from continuum solvation models.〔 The interaction operators have a clear meaning and are physically well defined. 1st - cavity creation; a term accounting for the energy spent to build a cavity in the solvent of suitable size and shape as to house the solute. Physically, this is energy cost of compressing the solvents structure when creating a void in the solvent. 2nd term - electrostatic energy; This term deals with the polarisation of the solute and solvent. 3rd term - an approximation for the quantum mechanical exchange repulsion; given the implicit solvent this term can only be approximated against high level theoretical calculations. 4th term - quantum mechanical dispersion energy; can be approximated using an averaging procedure for the solvent charge distribution.〔 These models can make useful contributions when the solvent being modelled can be modelled by a single function i.e. it is not varying significantly from the bulk. They can also be a useful way to include approximate solvent effects where the solvent is not an active constituent in the reaction or process. Additionally, if computer resources are limited, considerable computational resources can be saved by evoking the implicit solvent approximation instead of explicit solvent molecules. Implicit solvent models have been applied to model the solvent in computational investigations of reactions and to predict hydration free energy (ΔG(hyd)). Several standard models exist and have all been used successfully in a number of situations. The Polarizable continuum model (PCM) is a commonly used implicit model and has seeded the birth of several variants.〔 The model is based on the Poisson-Boltzmann equation, which is an expansion of the original Poisson's equation. Solvation Models (SMx) and the Solvation Model based on Density (SMD) have also seen wide spread use. SMx models (where x is an alphanumeric label to show the version) are based on the generalized Born equation. This is an approximation of Poisson's equation suitable for arbitrary cavity shapes. The SMD model solves the Poisson-Boltzmann equation analogously to PCM, but does so using a set of specifically parametrised radii which construct the cavity. The COSMO solvation model is another popular implicit solvation model. This model uses the scaled conductor boundary condition, which is a fast and robust approximation to the exact dielectric equations and reduces the outlying charge errors as compared to PCM. The approximations lead to a root mean square deviation in the order of 0.07 kcal/mol to the exact solutions. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Solvent models」の詳細全文を読む スポンサード リンク
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