We report a new structural model of silica molecular wire based on spiro union two-membered ring (SU-2MR) units. As revealed by density functional calculations, the SU-2MR wire is formed by parallel 2MRs bridged by oxygen atoms and is energetically more favorable, thermally more stable and chemically more reactive at the tip than the edge-sharing two-membered ring molecular chain proposed early. The SU-2MR molecular chain would be considered as an appropriate structural model of one-dimensional thin (0.4 nm) silica nanowires. (C) 2004 Elsevier B.V. All rights reserved.