A systematic understanding of orbital energy shift in polar solvent

The orbital energy of molecule is significantly shifted upon going from gas phase to solution phase. Based on Koopmans' theorem, the shift should be related to the change of ionization potential. However, the computed shift looks usually random and clear understanding has not been attained yet. Furthermore it is obtained only after solving complicated equations. In this study, we report a systematic framework for understanding the orbital energy shift by solvation effect and simple approximate formulae are presented.