Polarizable solvent model

Owing to its particular importance, polarizable solvent models have largely been restricted to water, for which a sizable number have been developed (see, for example, Dang 1992 Rick, Stuart, and Berne 1994 Bernardo et al. 1994 Zhu and Wong 1994 Lefohn, Ovchinnikov, and Voth 2001). Because evaluating the terms deriving from solvent polarizability... 

QM/MM approaches where the solute is QM and the solvent MM are in principle useful for computing the effect of the slow reaction field (represented by the solute point charges) but require a polarizable solvent model if electronic equilibration to the excited state is to be included (Gao 1994). With an MM solvent shell, it is no more possible to compute differential dispersion effects directly than for a continuum model. An option is to make the first solvent shell QM too, but computational costs for MC or MD simulations quickly expand with such a model. Large QM simulations with explicit solvent have appeared using the fast semiempirical INDO/S model to evaluate solvatochromic effects, and the results have been promising (Coutinho, Canute, and Zemer 1997 Coutinho and Canute 2003). Such simulations offer the potential to model solvent broadening accurately, since they can compute absorptions for an ensemble of solvent configurations. 

Gao, J., Energy components of aqueous solution Insight from hybrid QM/MM simulations using a polarizable solvent model. J. Comput. Chem. (1997) 18 1061—1071. 

Masella, M., Borgis, D., Cuniasse, P. Combining a polarizable force-field and a coarse-grained polarizable solvent model. II. accounting for hydrophobic effects. J. Comput Chem. 32, 2664-2678 (2011)... 

Lin ZX, Schmid N, van Gunsteren WF (2011) The effect of using a polarizable solvent model upon the folding equilibrium of different -peptides. Mol Phys 109 493-506... 

As can be seen from the histogram in Figure l-l(b), the loose conformation is preferred over the tight one, a result only possible with inclusion of solvent effects. Ab-initio calculations of those conformers show that, without the inclusion of solvent effects, the tight conformer is preferred by 7.4 kcal/mol, while the inclusion of solvent effects (with polarizable continuum model, PCM) shifts the preference towards the loose conformer, which becomes more stable than the tight one by 0.1 kcal/mol. 

C. Amovilli, V. Barone, R. Cammi, E. Cancfes, M. Cossi, B. Menucci, C. S. Pomelli, and J. Tomasi, Recent advances in the description of solvent effects with the polarizable continuum model, Adv. Quantum Chem. 32 227 (1998). 

The most common approach to solvation studies using an implicit solvent is to add a self-consistent reaction field (SCRF) term to an ab initio (or semi-empirical) calculation. One of the problems with SCRF methods is the number of different possible approaches. Orozco and Luque28 and Colominas et al27 found that 6-31G ab initio calculations with the polarizable continuum model (PCM) method of Miertius, Scrocco, and Tomasi (referred to in these papers as the MST method)45 gave results in reasonable agreement with the MD-FEP results, but the AM1-AMSOL method differed by a number of kJ/mol, and sometimes gave qualitatively wrong results. 

In addition to these external electric or magnetic field as a perturbation parameter, solvents can be another option. Solvents having different dielectric constants would mimic different field strengths. In the recent past, several solvent models have been used to understand the reactivity of chemical species [55,56]. The well-acclaimed review article on solvent effects can be exploited in this regard [57]. Different solvent models such as conductor-like screening model (COSMO), polarizable continuum model (PCM), effective fragment potential (EFP) model with mostly water as a solvent have been used in the above studies. 

The elucidation of actinide chemistry in solution is important for understanding actinide separation and for predicting actinide transport in the environment, particularly with respect to the safety of nuclear waste disposal.72,73 The uranyl CO + ion, for example, has received considerable interest because of its importance for environmental issues and its role as a computational benchmark system for higher actinides. Direct structural information on the coordination of uranyl in aqueous solution has been obtained mainly by extended X-ray absorption fine structure (EXAFS) measurements,74-76 whereas X-ray scattering studies of uranium and actinide solutions are more rare.77 Various ab initio studies of uranyl and related molecules, with a polarizable continuum model to mimic the solvent environment and/or a number of explicit water molecules, have been performed.78-82 We have performed a structural investigation of the carbonate system of dioxouranyl (VI) and (V), [U02(C03)3]4- and [U02(C03)3]5- in water.83 This study showed that only minor geometrical rearrangements occur upon the one-electron reduction of [U02(C03)3]4- to [U02(C03)3]5-, which supports the reversibility of this reduction. 

To answer this question, let us first consider a neutral molecule that is usually said to be polar if it possesses a dipole moment (the term dipolar would be more appropriate)1 . In solution, the solute-solvent interactions result not only from the permanent dipole moments of solute or solvent molecules, but also from their polarizabilities. Let us recall that the polarizability a of a spherical molecule is defined by means of the dipole m = E induced by an external electric field E in its own direction. Figure 7.1 shows the four major dielectric interactions (dipole-dipole, solute dipole-solvent polarizability, solute polarizability-solvent dipole, polarizability-polarizability). Analytical expressions of the corresponding energy terms can be derived within the simple model of spherical-centered dipoles in isotropically polarizable spheres (Suppan, 1990). These four non-specific dielectric in-... 

A mean field theory for the stndy of solvent effects considers polarizable solvent molecules. The model, which combines qnantnm chemistry and simnlation calculations, splits the system into three parts ... 

Since the Tomassi et a/.86,87 Polarizable Continuum Model, PCM, to describe dielectric solvent effects is implemented within the Gaussian9881 suite of programs, Peralta and Barone s modified version of the Gaussian98 suite of programs can be used to calculate solvent effects on the FC, SD and DSO terms (however for the FC term the perturbative FPT approach should be used if solvent effects are to be calculated). 

In general, however, the majority of properties do not yet seem to be more accurately predicted by polaiizable models than by unpolarizable ones, provided adequate care is taken in the parameterization process. Of course, if one wishes to examine issues associated with polarization, it must necessarily be included in the model. In the area of solvents, for instance, Bernardo et al. (1994) and Zhu and Wong (1994) have carefully studied the properties of polarizable water models. In addition, Gao, Habibollazadeh, and Shao (1995) have developed... 

At the next level of complexity, the polarity of solvent models, as made manifest by their atomic partial charges, can be augmented with a polarizability. This allows the solvent molecule to respond to its surroundings in a fashion conceptually similar to the electronic component of die solvent polarization described in Section 11.1.1. Typically a polarizability tensor a is assigned either to the solvent molecule as a whole or to individual atoms. Then, die induced dipole moment at each polarizable position can be determined from... 

---