Solvation explicit/implicit hybrid models

So, while there is growing interest in hybrid models of all sorts (as discussed in more detail in the next chapter), the choice of a mixed solvent model is not necessarily intrinsically better than a pure explicit or pure implicit model. In general, unless there is a strong suspicion that first-solvation-shell effects are drastically different from those more typically encountered, there is no particularly compelling reason to pursue a mixed modeling strategy. An example... 

Okur, A., Wickstrom, L., Layten, M., Geney, R., Song, K., Hornak, V., Simmerling, C. Improved efficiency of replica exchange simulations through use of a hybrid explicit /implicit solvation model. 

In this framework, a generalization of a hybrid explicit/implicit solvation model for the treatment of polarizable molecular systems at different levels of theory has been recently proposed by our group, the so-called GLOB model [33, 34]. Such a... 

Solvation can be included in calculations implicitly (e.g., in PB-SCRF, PCM/DIR, SM2, and other continuum solvent models that emulate properties of bulk solvent at varying degrees of sophistication) or explicitly (by placing solvent molecules around the solute). The former approach is arguably more widely used, probably owing to the computational efficiency of implicit solvation and the avoidance of the compUcating issue of explicit solvent placement. Nevertheless, the sacrifice of atomic detail for the solvent is not always justified and recently, a hybrid explicit/implicit solvation method was proposed, treating the first solvation shell in atomic detail and the remainder of the solvent with a SCRF model [29]. 

This hybrid solvation model surrounds the solute with a small number of explicit solvent molecules, and then embeds this cluster into the implicit dielectric field. Local effects are addressed by the full quantum mechanical treatment of the interaction between the solute and the few explicit solvent molecules. Long-range effects are included through the interaction of the cluster with the dielectric field. A decision is still needed as to how many explicit solvent molecules should be included within the cluster, recognizing that each additional solvent molecule increases the size of the calculation and expands the configurational space that must be explored. 

Going beyond the mean-field level, several "hybrid" approaches are now being explored in MD simulations. Examples include a recent model [70] in which the immediate hydration of the solute is modeled explicitly by a layer of water molecules, and the GB model is used to treat the bulk continuum solvent outside the explicit simulation volume. A similar idea was recently found very effective in the context of replica-exchange simulations [71]. An explicit ion/implicit water (PB) solvation model for molecular dynamics of nucleic acids has recently been tested [72]. 

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