Strengths and Weaknesses of Continuum Solvation Models

There are certain instances where this approach may be regarded as an attractive option. For example, Cossi and Crescenzi (2003) found that accurate computation of NMR chemical shifts for alcohols, etliers, and carbonyls in aqueous solution required at least one explicit solvent shell, but that beyond that shell a continuum could be used to replace what would otherwise be a need for a much larger cluster. However, just as the strengths of the two models are combined, so are the weaknesses. A typical first shell of solvent for a small molecule may be expected to be composed of a dozen or so solvent molecules. The resulting supermolecular cluster will inevitably be characterized by a large number of accessible structures that are local minima on the cluster PES, so that statistical sampling will have to be undertaken to obtain a proper equilibrium distribution. Thus, QM methods require a substantial investment of computational resources. In addition, certain technical points require attention, e.g., how does one keep the first solvent shell from exchanging with the continuum since both, in principle, foster identical solvation interactions ... 

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