The electronic spectrum of the micro-solvated alanine zwitterion calculated using the combined coupled cluster/molecular mechanics method

We present a theoretical study of the vertical electronic excitation energies of the alanine zwitterion in an aqueous environment. The study is based on calculations using the combined coupled cluster/molecular mechanics (CC/MM) method. The CC/MM results (using different water potentials) are compared to calculations based on density functional theory (DFT) using the B3LYP functional. Special attention is payed to the lowest n O → π CO ∗ transition and higher lying states including the π CO → π CO ∗ . We find that the spectra obtained using CC/MM are in good agreement with experiments. On the other hand, less satisfactory results are found for the spectrum calculated using DFT.