VBEFP/PCM: a QM/MM/PCM approach for valence-bond method and its application for the vertical excitations of formaldehyde and acetone in aqueous solution

Abstract

In this paper, a combined QM/MM/PCM approach, named VBEFP/PCM, is presented for ab initio VB study with a solvent effect incorporated. In VBEFP/PCM, both short-range and long-range solvent effects are taken into account by effective fragment potential (EFP) and polarizable continuum model (PCM), respectively, while the solute molecules are described by valence bond (VB) wave function. Furthermore, VBEFP/PCM, along with VBPCM and VBEFP, is employed for the n→π* vertical excitation of formaldehyde and acetone molecules in aqueous solution. The computational results show that VBEFP/PCM can provide the appropriate solvent shifts, whereas VBPCM underestimates the solvent shifts due to its lack of short-range solvent effect. The VBEFP results strongly rely upon the description of the short-range solvent effect. To explore the role of the solute’s electronic structure in the solvent shift, resonance energy analysis during the excitation is performed. It was found that the solute’s electronic polarization plays the most important role in the solvent shift. The resonance controls the variation of the solute’s wave function during the n→* vertical excitation, which leads to the blue solvent shifts.