The n→π* electronic transition in microsolvated formaldehyde.: A coupled cluster and combined coupled cluster/molecular mechanics study

In this article, we study the n - pi* electronic transition in aqueous microsolvated formaldehyde using the coupled cluster (CC) and coupled cluster/molecular mechanics (CC/MM) methods. The CC models used are the coupled cluster singles and doubles (CCSD) and the coupled cluster second-order approximate singles and doubles (CC2) methods. The CC/MM model includes electrostatic and mutual polarization effects on the calculated electronic excitation energies. The CC/MM shifts of the lowest electronic excitation energy compare successfully to the corresponding shifts as defined in the supermolecular approach. Finally, we include, in addition to the explicit water molecules in the supermolecular calculations, a dielectric medium to account for the long-range interactions. The result for the shift in electronic excitation energy compares well with both other theoretical approaches and available experimental data.