TDDFT Study of the Optical Absorption Spectra of Bare and Coated Au55 and Au69 Clusters

The optical absorption of bare and ligand-coated Au-55 and Au-69 "Schmid" clusters was calculated using time-dependent density functional theory. Calculations were performed using the explicit time propagation method with the local density approximation for the exchange-correlation potential. Both icosahedral and cuboctahedral structures of the Au-55 gold core were simulated. The ligand coating was shown to have the effect of reducing the features of the optical absorption spectrum of the clusters, giving a profile more similar to experimental results. The difference in the optical absorption between the different geometries and core sizes is also less marked when the clusters are coated. The results suggest that within the 1.4 nm size range, the absorption spectra are dominated by the coating and are not experimentally distinguishable. Binding energies were also calculated for the Au-55 cluster, showing that the cuboctahedral structure has lower energy although the energy difference is very small. The effect of the coating on the electron density of the gold cluster is also investigated by subtracting the electron densities of the bare clusters from those of the coated clusters.