We report the development of an ab initio scheme designed for the calculation of the electronic ground state and low-lying excited states of an atom or a molecule, perturbed by a weakly interacting environment of discrete, unpolarizable particles acting as a solvent. The model employs an ab initio partitioning ansatz that accounts for the effects of nonlocal exchange-overlap interactions between the solute and the solvent by means of a parametrized exchange-overlap operator and an effective metric in the pair-permutation, pair-additivity approximation, which is known to be valid in regions of small intermolecular overlap. Intramolecular perturbations like spin-orbit effects can be incorporated in the treatment. Due to its fast performance and built-in size-consistency, our model can be employed in the calculation of the electronic states of spectroscopically active fragments with many different settings of the environment.