Photoabsorption Spectra of (SiO₂)_n (n≤5) Clusters on the Basis of Time-Dependent Density Functional Theory

The photoabsorption spectra of (SiO₂)_n (n=2-5) clusters including isomers (D_3h,D_2d) structures of (SiO₂)₃ and (C_2v,D_2h,D_4h) structures of (SiO₂)₄ are calculated by using time-dependent density-function theory. The equilibrium geometries, the binding energy, the gap between the highest occupied and lowest unoccupied molecular orbitals and vertical ionization potential for corresponding structures are computed using several methods with different types of the basis functions. It is found that the polarizability functions are necessary for the basis functions when optimize the structures of silicon oxide clusters. For different geometries of various clusters and the related isomers, their spectra are very different. Meanwhile, the comparison between using local-density generalized-gradient approximations for exchange-correlation potentials shows that both the calculated spectra present the same spectral feature. We suggest that the calculated photoabsorption spectra could be taken as a tool to elucidate the isomers and clusters structure.