An explicit quantum chemical solvent model for strongly coupled solute–solvent systems in ground or excited state

Abstract

A detailed account of the explicit quantum chemical solvent model QMSTAT is given. The model is presented in terms of three coupled aspects of relevance for all types of quantum chemical solvent models: The quantum chemical method, the intermolecular interactions and the statistical mechanical method. The quantum chemical method is either a compact natural orbital formulation of the standard Hartree–Fock method or a compact multiconfigurational method with a state basis. The latter method can describe excited states apart from the ground state and is for most systems an excellent approximation to the complete active space self-consistent field method. Both static and induced electrostatic interaction terms between the quantum chemical region and the solvent are included. Further, a non-electrostatic term is added to describe effects which derive from the Pauli principle. This term models both the exchange repulsion between solute and solvent and the packing effects an environment has on a molecule, in particular on diffuse states of the molecule. The statistical mechanical problem is solved with an exact Metropolis–Monte Carlo simulation that requires several similar quantum chemical problems to be solved. Since the quantum chemical problem and the statistical mechanical problem are solved as a coupled problem, the present model is especially useful for problems where electronic degrees of freedom of the solute strongly depend on the solvent distribution and vice versa. Three applications are summarized, which highlight this type of coupling present in QMSTAT and the non-electrostatic contribution. The examples are the solvation of four monatomic ions, the solvation of para-benzoquinone and the solvation of indole and the solvent shift to its absorption and fluorescence spectra.