Optical Properties of Au Nanoclusters from TD-DFT Calculations

A time-dependent density-functional-theory (TD-DFT) approach is employed to investigate theoretically the optical response of Au nanoclusters of size around N = 150 atoms as a function of: (a) the approximation used for the DFT exchange-correlation (xc-) functional, (b) the shape of the nanocluster. The results of the local-density-approximation (LDA) and the van Leeuwen-Baerends (LB94) xc-functionals are compared on a set of 4 structural motifs: octahedral (N= 146), cuboctahedral (N= 147), icosahedral (N = 147), and cubic (N= 172), representative of both crystalline and noncrystalline motifs commonly encountered in the study of metal nanoclusters. It is found that the position of the peak in the photoabsorption spectrum is weakly dependent on the shape of the cluster but is strictly related to its size and to the DFT xc-functional used in the calculations, with the finding that the predictions of the LB94 xc-functional compare better with the available experimental data on the absorption spectrum of Au particles in this size range with respect to those of the LDA xc-functional. The detailed shape of the cluster becomes apparent in the form of the absorption spectrum, which can be symmetric or asymmetric in two different forms.