Generalized molecular mechanics including quantum electronic structure variation of polar solvents. I. Theoretical formulation via a truncated adiabatic basis set description

A theoretical framework to describe the evolving electronic charge distributions of bulk solvent that can be incorporated into the computer simulation algorithms is considered. By using a truncated adiabatic basis set representation, an effective quantum mechanical description for the electronic polarizabilities of the constituent molecules is constructed. It is found that there is a near one-to-one correspondence between the theory developed here and gas-phase quantum chemistry; with the substitution, electrons $\to$ molecules, many known results of the latter are transferable to the former with some modifications. The ground-state solution for the solvent system is studied with the analogs of the self-consistent field (SCF) and second-order Mo/ller–Plesset (MP2) methods of many-body perturbation theory. It is found that the states with one of the molecules electronically excited are not directly coupled to the SCF ground state; this is very similar to Brillouin’s theorem in the gas-phase electronic structure theory. As a result, they do not contribute to the MP2 energy correction; only the two-molecule excited states make nonvanishing contributions. The ground-state electronic properties, e.g., total and single-molecule polarizability tensors, are analyzed at the SCF level. The inclusion of electronic relaxation in the excited state calculations is briefly considered within the context of the configuration interaction method. The incorporation of the theory into the molecular dynamics computer simulation algorithms via the interaction site model description is also discussed.