Electronic excitation energies ofZniSiclusters

Abstract

Time-dependent density-functional theory (TDDFT) excitation energies are calculated for ${\mathrm{Zn}}_{i}{\mathrm{S}}_{i}$ global minima clusters, $i=1--9.$ The geometry of the global minima is ringlike for $i=1--5$ and three-dimensional (3D) spheroidlike for $i=6--9.$ In general, the calculated excitations happen from nonbonding p orbitals of sulfur. These orbitals are perpendicular to the molecular plane in the case of the rings, and normal to the spheroid surface for 3D clusters. The calculated excitation energies are larger for ringlike clusters as compared to 3D ones, with the excitation energies of the latter structures lying close to the visible spectrum. The difference between Kohn-Sham eigenvalues of the orbitals involved in the electronic excitations studied have also been compared with the TDDFT results of the corresponding excitations for two approximate density functionals, i.e., MPW1PW91 and B3LYP, the latter being more accurate. The B3LYP excitation energies calculated as the difference between Kohn-Sham eigenvalues of the orbitals involved in the excitation have been found to be only 0.30--0.40 eV too high for the smaller 3D-like clusters. Moreover, they approach the TDDFT value as the cluster size increases. Therefore, this might be a practical approach to estimate excitation energies of large ${\mathrm{Zn}}_{i}{\mathrm{S}}_{i}$ clusters.