Explicit solvent effects

Contributing to Ajj are, in addition to the solvent structural effects explicitly considered, contributions from dielectric saturation, from the liquid structure effects one has even in simple fluids, from solvent-mediated dispersion interactions of the ions, from charge-polarizability interactions of the ions, and so on. It is difficult to tell a-priori which effects are dominant or how big they are. However the collection of A 5 coefficients has characteristics that are consistent with the first named effect being dominant. 

In particular, V° describes a solute-solute Coulomb and exchange-correlation interaction corrected by an overlap contribution. The effects of the solvent on V° are implicitly included in the values of the transition properties of the two chromophores before the interaction between the two is switched on. These properties can in fact be significantly modified by the reaction field produced by the polarized solvent. In addition, the solvent explicitly enters into the definition of the coupling through the term VIEF of Equation (3.150), which describes the chromophore-solvent-chromophore interaction. 

The effect of the medium denoted here as implicit reflects the influence of the solvent on the transition densities (i.e., spectral properties) of the D/A units, which determine the direct coupling Vs. The solvent explicitly enters into the definition of the coupling through the term T Xpiicit in (2.4), which describes an interaction between the two chromophores mediated by the medium, that generally leads to an overall reduction (i.e., a screening) of the D/A coupling. 

QMSTAT is an effective quantum chemical solvent model with an explicit solvent representation. Effective here means that the quantum chemical electronic Hamiltonian only pertains to a small subset of the total system (typically the solute), with the solvent entering as a perturbation operator to the Hamiltonian explicit solvent means that the solvent is described with a set of spatial coordinates and parametrized physical features significantly simplified compared to a full quantum chemical description. The explicit solvent representation implies that it is possible to go beyond the mean-field approximation inherent in the often used continuum... 

Empirical Valence Bond Methods. - To examine some important questions relating to enzyme action (e.g. to analyse the causes of catalysis, i.e. why an enzymic reaction proceeds faster than the equivalent, uncatalysed reaction in solution), it is necessary to use a method that not only captures the essential details of the chemical reaction, but also includes the explicit effects of the enzyme and solvent enviroment. One notable method in this area is the empirical valence bond (EVB) model.143 In the empirical valence bond approach, resonance structures (for example ionic and covalent resonance forms)... 

As computer power has increased it has become possible to incorporate explicitly some solvent molecules and thereby simulate a more realistic system. The simplest way to do this is to surroimd the molecule with a skin of solvent molecules. If the skin is sufficiently deep then the system is equivalent to a solute molecule inside a drop of solvent. The number of solvent molecules in such cases is usually significantly fewer than would be required in the analogous periodic boundary simulation, where the solute molecule is positioned at the centre of the ceU and the empty space is filled with solvent. Boundary effects should be transferred from the molecule-vacuum interface to the solvent-vacuum interface and so might be expected to result in a more realistic treatment of the solute. To illustrate these three situations, we can consider dihydrofolate reductase, which is a small enzyme that contains approximately 2500 atoms. If this enzyme is surrounded by water molecules in a cubic periodic system such that the surface of the protein is at least 10 A from any side of the box, then the number of atoms rises to almost 20000. If a shell 10 A thick is used then the number of atoms falls to 14 700, and with a 5 A shell the system contains 8900 atoms. 

In this approach we neglected the explicit effect of the solvent in the reaction coordinate. In fact, this effect can be incorporated in the parameters X or n and, to a smaller extent, in the force constants. ISM predicts that the effect of the solvent on the ET rates can be small, as apparent from the similar energy barriers for the electron exchange of alkylhydrazines in acetonitrile and in the gas phase. ... 

Langevin dynamics is a method to simulate molecules in contact with a heat bath or solvent without considering the explicit structure of the solvent. An important application of this approach is conformation sampling for macromolecules in solution, because the expensive integration of the solvent trajectories is omitted—as well as solvent specific effects of course. Here the equations of motion are given by... 

An alternative approach is to combine QM and MM methods such that the reacting system (or the active site in an enzyme) is treated explicitly by a quantum mechanical method, while the surrounding environmental solvent molecules (or amino acids), which constitute the most time-consuming part in the evaluation of the potential energy surface, are approximated by a standard MM force field. " Such a method takes advantage of the accuracy and generality of the QM treatment for chemical reactions - and of the computational efficiency of the MM calculation.Because the reactant electronic structure and solute-solvent interactions are determined quantum mechanically, the procedure is appropriate for studying chemical reactions, and there is no need to parameterize potential functions for every new reaction. Furthermore, the solvent polarization effects on the solute are naturally included in the... 

However, description of the mutual solute—solvent polarization effects for both the ground and the exdted states within molecular orbital theory is not straightforward, because it requires SCF calculations of the solvent-induced dipoles in each step of the HF—SCF interaction. A simplification can be made by treadng the solvent molecule with fixed, efifeaive partial charges (thus ignoring explicit solvent polarization effects). In this case, = Eg, and the calculated spectral shift may be further decomposed into two terms ... 

---