Characterization of optical spectra of interacting systems: Application to oxide-supported metal clusters

We present a general strategy for interpreting optical spectra of interacting systems on the basis of linear-response time-dependent density functional theory (TDDFT) calculations. For this purpose, we combined a natural transition orbitals (NTO) analysis with a fragment molecular orbital (FMO) analysis. The NTO transformation allows one to identify leading contributions of particle-hole excitations in the spectral bands, whereas the FMO analysis characterizes each pair in terms of (leading) contributions of the corresponding fragments. We applied this procedure to assign and characterize optical transitions of coinage metal dimers M₂ (M = Cu, Ag, Au) adsorbed at regular oxygen sites of MgO(001) as well as at oxygen vacancies, F_s and F_s⁺. The TDDFT calculations were carried out at the generalized-gradient level on structures that had been obtained with cluster models embedded in an elastic polarizable environment. The combined NTO and FMO results allowed us to analyze the spectra both qualitatively and quantitatively. The qualitative results agree very well with a previous assignment carried out by our group (Bosko et al., J Phys Chem A, 2007, 111, 6870), whereas the present approach yields a straightforward and transparent quantitative characterization of the main spectral bands.