Solute-solvent polarization

In the Generalized Born model [2-5], the solvent is described in a extremely simplified way and there is no mutual polarization between solute and solvent. The Onsager model [6] allows for solute-solvent polarization, but the description of the cavity and of the solvent is still very crude. 

M.A. Thompson, QM/MMpol A consistent model for solute/solvent polarization. application to the aqueous solvation and spectroscopy of formaldehyde, acetaldehyde, and acetone, J. Phys. Chem., 100 (1996), 14492-14507. 

Suppose a donor-acceptor (DA) pair is in the charge state which is labelled as D A° in the figure. The unit of the charges is the electronic charge e. Solvent around the solute pair will have an equilibrium polarization corresponding to the electrostatic field from the solute. Solvent polarization is represented by the orientation of the thick arrows. The solvent polarization will be caused to deviate from... 

C. M. Hansen, in Macromolectdar Solutions Solvent-Polarity Relationships in Polymers, Proceedings of the Symposium, American Chemical Society, New York, 1981, edited by R. B. Seymour and G. A. Stahl (Pergamon, New York, 1982), p. 1 Org. Coat. Plast. Chem. 45, 227 (1981). 

Gao et al. worked out QM/MM-based methods to consider solvent effects on chemical reactions. The authors combine semiempirical and ab initio calculations applied to the QM region with the so-called polarizable MM approach for the MM region. The latter includes mutual solute-solvent polarization interaction which the authors show to be significant. [Note that strong polarization effects means that the second order perturbation QM-MM interaction is large which, in turn, indicates that the this interaction may be not small, see Section II]. 

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 ... 

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