A systematic study of the geometric structures of steady states and metastable states of silicon oxide clusters has been performed using density functional theory. We find that silicon-rich and oxygen-rich clusters have different characteristics. Oxygen-rich clusters usually have oxygen atoms on the edges of the clusters, but separated from others by Si atoms. However, silicon-rich clusters tend to have rings nested within each other. The spectra for the structures have been calculated to compare with observed spectra. The predicted structures and spectroscopic properties are expected to be useful for the identification of silicon oxide species in the interstellar medium. (C) 2015 Elsevier B.V. All rights reserved.