Bond-stretch isomerism is predicted not to occur in the parent compound Si4H6 and the substituted 1,3-di-tert-butyl derivative, although the existence of bond-stretch isomerism in the 1,3-dimethyl derivative is a possibility. Bulky bridgehead moieties induce a preference for a short Si−Si bridge bond, while a longer bridge bond length (2.840 Å at the MP2/6-31G(d,p) level) is predicted for the unsubstituted molecule.