A fully quantum mechanical simulation study on the lowest n–π∗ state of hydrated formaldehyde

We report a fully quantum mechanical simulation on the lowest n–π∗ excited state of the hydrated formaldehyde, by using the fragment molecular orbital (FMO) scheme. For a droplet cluster consisting of a formaldehyde molecule and 128 water molecules, the molecular dynamics (MD) simulation was directly carried out by the FMO-based Hartree–Fock/6-31G energy gradient. The FMO-MD run generated a series of configuration samples with geometrical fluctuation. These samples were then subjected to the excited state calculation of configuration interaction singles with perturbative doubles (CIS(D)) in the multilayer FMO (MLFMO) framework. The final estimation of the excitation energy was obtained to be 4.22 eV through averaging the MLFMO-CIS(D)/6-31G∗ energies.