Self-consistent field calculations, solute-solvent interaction

In the RISM-SCF theory, the statistical solvent distribution around the solute is determined by the electronic structure of the solute, whereas the electronic strucmre of the solute is influenced by the surrounding solvent distribution. Therefore, the ab initio MO calculation and the RISM equation must be solved in a self-consistent manner. It is noted that SCF (self-consistent field) applies not only to the electronic structure calculation but to the whole system, e.g., a self-consistent treatment of electronic structure and solvent distribution. The MO part of the method can be readily extended to the more sophisticated levels beyond Hartree-Fock (HF), such as configuration interaction (Cl) and coupled cluster (CC). 

Self-consistent reaction field (SCRF) models are the most efficient way to include condensed-phase effects into quantum mechanical calculations [8-11]. This is accomplished by using SCRF approach for the electrostatic component. By design, it considers only one physical effect accompanying the insertion of a solute in a solvent, namely, the bulk polarization of the solvent by the mean field of the solute. This approach efficiently takes into account the long range solute-solvent electrostatic interaction and effect of solvent polarization. However, by design, this model cannot describe local solute-solvent interactions. 

In the implicit approach, as in the SCRF (self-consistent field reaction) method,25 the solvent is treated as a continuum whose principal characteristic is its dielectric constant. The solute is placed in a cavity within the solvent, which becomes polarized by its presence. In turn, this creates an electric field within the cavity. Subsequently, the free energy of solvation is calculated by a multipolar development. This method does not require much computer time. It ignores, however, specific solvent-solute interactions such as hydrogen bonds. 

Continuum solvation models consider the solvent as a homogeneous, isotropic, linear dielectric medium [104], The solute is considered to occupy a cavity in this medium. The ability of a bulk dielectric medium to be polarized and hence to exert an electric field back on the solute (this field is called the reaction field) is determined by the dielectric constant. The dielectric constant depends on the frequency of the applied field, and for equilibrium solvation we use the static dielectric constant that corresponds to a slowly changing field. In order to obtain accurate results, the solute charge distribution should be optimized in the presence of the field (the reaction field) exerted back on the solute by the dielectric medium. This is usually done by a quantum mechanical molecular orbital calculation called a self-consistent reaction field (SCRF) calculation, which is iterative since the reaction field depends on the distortion of the solute wave function and vice versa. While the assumption of linear homogeneous response is adequate for the solvent molecules at distant positions, it is a poor representation for the solute-solvent interaction in the first solvation shell. In this case, the solute sees the atomic-scale charge distribution of the solvent molecules and polarizes nonlinearly and system specifically on an atomic scale (see Figure 3.9). More generally, one could say that the breakdown of the linear response approximation is connected with the fact that the liquid medium is structured [105],... 

The methodology that uses the dielectric model is instead the simpler and in principle the more suitable for the study of chemical reactions involving large molecular systems. In 1998, Amovilli et al [13] developed a computer code in which the solvent reaction field, including all the basic solute-solvent interactions, has been considered for Complete Active Space Self Consistent Field (CASSCF) calculations. 

Proper inclusion of the solvent into the calculations is unfortunately quite difficult [46]. One can use classical molecular dynamics or Monte Carlo simulations, classical continuum models based on the Poisson-Boltzman equation, and quantum-chemical studies using various variants of the Self Consistent Reaction Field (SCRF) approach at the semiempirical or ab initio level. There are serious approximations associated with these methods. Continuous models neglect the specific solute-solvent interactions which are very important for polar solvent. Classical methods neglect the changes in the electronic structure of the solute due to the solvent effects. These uncertainties can be illustrated using the predicted solvation energy of adenine treated by various modem approaches. The calculated values vary from -8 to -20 kcal/mol [68]. 

The study of solvatochromic shifts is of great importance and has received enormous theoretical attention in recent years. Progress has been achieved in the use of the self-consistent reaction field and cavity models. These advances have also shown several limitations. It is thus of great interest to have alternative procedures to calculate solvent effects. In this respect the use of Monte Carlo/Molecular Dynamics simulations has been growing. In this paper we suggest a procedure to allow a full quantum mechanical calculation of the solute-solvent system. The basic idea is to treat the solute, the solvent and its interaction by quantum mechanics. First a Monte Carlo simulation is performed to characterize the liquid structure. These structures are then used in the quantum mechanical calculation. As a liquid has not one but a great number of structures equally possible within a... 

Colominas et al. [143] published a very detailed study on the dimerization of formic and acetic acids in the gas phase and in aqueous and chloroform solutions. By using quantum mechanical self-consistent reaction field and Monte Carlo calculations, they showed that the dimerization is favored in the gas phase and in a chloroform solution (1M) basically due to double hydrogen-bonded interaction between the monomers. In aqueous solution, the dimerization does not seem to occur due to the competitive solute-solvent intermolecular interactions. The computed dimerization energies are shown in Table VII. 15... 

Reaction field methods model solutions by placing the solute in a cavity of a polarizable medium. The electrostatic potential due to the solute molecule polarizes the surrounding medium which in turn changes the charge distribution of the solute. Hence, the electrostatic interaction has to be evaluated self-consistently (self-consistent reaction field, SCRF). A term for creating the cavity (calculated from the surface of the cavity) has a be added to the solvation energy. Explicit treatment of solvent molecules can be combined with a reaction field method. 

Solvent potential. The averaged solvent electrostatic field, , is important for inhomogeneous media, such as enzymes, membranes, miscelles and crystalline environments systems. Due to the existence of strong correlations, such a field does not cancel out. This factor becomes an important contribution to solvent effects at a microscopic level. In a study of non-rigid molecules in solution, Sese et al. [25] constructed a by using the solute-solvent atom-atom radial distribution function. Electrostatic interactions in three-dimensional solids were treated by Angyan and Silvi [26] in their self-consistent Madelung potential approach such a procedure can be traced back to a calculation of . An earlier application of the ISCRF theory to the study of proton mechanisms in crystals of hydronium perchlorate both [Pg.441]

---