Continuum solvation

E. Cances and B. Mennucci, New applications of integral equation methods for solvation continuum models ionic solutions and liquid crystals, J. Math. Chem., 23 (1998) 309. 

E. Cances and B. Mennucci, The escaped charge problem in solvation continuum models, J. Chem. Phys., 115 (2001) 6130. 

However, even simple liquids have short-range order around the solute, i.e., in the region that is most important in determining solute-solvent interactions, and this order does not prevent solvation continuum models from being quite successful. 

For the first time, solvation continuum models are treated in an up-to-date and coherent way but at the same time using very different points of view coming from experts belonging to very different research fields (mathematicians, theoretical chemists, computational chemists, spectroscopists, etc.). 

The study can be summarized as a two-step procedure. The first step is an analysis of the solvent-induced modifications on the property of interest, when obtained through a solvation continuum model. At this level it is fundamental that the continuum model is as accurate as possible in our case this is realized through the PCM approach. On the basis of the results obtained in this continuum framework... 

It can be anticipated that the computation of A//soi and AAsoi is more delicate than the prediction of AGsoi, which benefits from the enthalpy-entropy compensation. Accordingly, the suitability of the QM-SCRF models to predict the enthalpic and entropic components of the free energy of solvation is a challenging issue, which could serve to refine current solvation continuum models. This contribution reports the results obtained in the framework of the MST solvation model [15] to estimate the enthalpy (and entropy) of hydration for a set of neutral compounds. To this end, we will first describe the formalism used to determine the MST solvation free energy and its enthalpic component. Then, solvation free energies and enthalpies for a series of typical neutral solutes will be presented and analyzed in light of the available experimental data. Finally, collected data will be used to discuss the differential trends of the solvation in water. 

In this section we present two studies on energy properties of solvated systems. In the first we shall analyze the solvent effects on the stability of isomers of glycine and alanine in water solution, whereas in the second we sheill analyze the solvent induced changes on the structures of donor-acceptor conjugated systems. The two studies represent two examples of application of solvation continuum models in two different important fields... 

Numerous theoretical studies on DMABN have been carried out, and many of them confirm the greater validity of the TICT model. The main body of such calculations, however, has been limited to the isolated system, while few examples including solvent effects can be quoted. " On the contrary, the phenomenon is strongly related to solvation and thus explicit considerations of solvent interactions are very important to get a more accurate understamding of the experimental evidence on the specific effects due to the presence of polar solvents. Here we summarize the results of the correlated study of DMABN both in vacuo and in solution we have published on the Journal of American Chemical Society. In this study we have used the multireference perturbation configuration interaction (Cl) method, known with the CIPSI acronym, which has been coupled to the PCM-IEF solvation continuum model. ... 

The FI2O molecules of these aquo-complexes constitute the iimer solvation shell of the ions, which are, in turn, surrounded by an external solvation shell of more or less uncoordinated water molecules fomiing part of the water continuum, as described in section A2.4.2 above. Owing to the difference in the solvation energies,... 

Within the framework of the same dielectric continuum model for the solvent, the Gibbs free energy of solvation of an ion of radius and charge may be estimated by calculating the electrostatic work done when hypothetically charging a sphere at constant radius from q = 0 q = This yields the Bom equation [13]... 

Cramer C J and Truhlar D G 1996 Continuum solvation models Solvent Effects and Ohemical Reactivity ed O Tapia and J Bertran (Dordrecht Kluwer) pp 1-80... 

Mineva T, Russo N and Sicilia E 1998 Solvation effects on reaction profiles by the polarizable continuum model coupled with Gaussian density functional method J. Oomp. Ohem. 19 290-9... 

Wood, R.H. Continuum electrostatics in a computational universe with finite cut-off radii and periodic boundary conditions Correction to computed free energies of ionic solvation. J. Chem. Phys. 103 (1995) 6177-6187. 

It is often the case that the solvent acts as a bulk medium, which affects the solute mainly by its dielectric properties. Therefore, as in the case of electrostatic shielding presented above, explicitly defined solvent molecules do not have to be present. In fact, the bulk can be considered as perturbing the molecule in the gas phase , leading to so-called continuum solvent models [14, 15]. To represent the electrostatic contribution to the free energy of solvation, the generalized Bom (GB) method is widely used. Wilhin the GB equation, AG equals the difference between and the vacuum Coulomb energy (Eq. (38)) ... 

Another way of calculating the electrostatic component of solvation uses the Poisson-Boltzmann equations [22, 23]. This formalism, which is also frequently applied to biological macromolecules, treats the solvent as a high-dielectric continuum, whereas the solute is considered as an array of point charges in a constant, low-dielectric medium. Changes of the potential within a medium with the dielectric constant e can be related to the charge density p according to the Poisson equation (Eq. (41)). 

Tiic Langevin dipole method of Warshel and Levitt [Warshel and Levitt 1976] i.itermediate between a continuum and an explicit solvation model. A three-dimension... 

The final class of methods that we shall consider for calculating the electrostatic compone of the solvation free energy are based upon the Poisson or the Poisson-Boltzmann equatior Ihese methods have been particularly useful for investigating the electrostatic properties biological macromolecules such as proteins and DNA. The solute is treated as a body of co stant low dielectric (usually between 2 and 4), and the solvent is modelled as a continuum high dielectric. The Poisson equation relates the variation in the potential (f> within a mediu of uniform dielectric constant e to the charge density p ... 

Cramer C J and Truhlar D G 1995. Continuum Solvation Models Classical and Quantum Mechanical Implementations. In Lipkowitz K B and D B Boyd (Editors) Reviews in Computational Chemistry Volume 6. New York, VCH Publishers, pp. 1-72. 

Qiu D, P S Shenl F P HoUinger and W C Still 1997. The GB/SA Continuum Model for Solvation. / Fast Anal5dical Method for the Calculation of Approximate Bom Radii. Journal of Physical Chcniistr 101 3005-3014. 

It is sometimes desirable to include the effect of the rest of the system, outside of the QM and MM regions. One way to do this is using periodic boundary conditions, as is done in liquid-state simulations. Some researchers have defined a potential that is intended to reproduce the effect of the bulk solvent. This solvent potential may be defined just for this type of calculation, or it may be a continuum solvation model as described in the next chapter. For solids, a set of point charges, called a Madelung potential, is often used. 

There is no one best method for describing solvent effects. The choice of method is dependent on the size of the molecule, type of solvent effects being examined, and required accuracy of results. Many of the continuum solvation methods predict solvation energy more accurately for neutral molecules than for ions. The following is a list of preferred methods, with those resulting in the highest accuracy and the least amount of computational effort appearing first ... 

OPW (orthogonalized plane wave) a band-structure computation method P89 (Perdew 1986) a gradient corrected DFT method parallel computer a computer with more than one CPU Pariser-Parr-Pople (PPP) a simple semiempirical method PCM (polarized continuum method) method for including solvation effects in ah initio calculations... 

SCF (self-consistent field) procedure for solving the Hartree-Fock equations SCI-PCM (self-consistent isosurface-polarized continuum method) an ah initio solvation method... 

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