The structures of isolated, alpha,alpha-linked oligothiophene dications [H-(C4H2S)(n)-H](2+) have been computed at the ab initio SCF level for short and intermediate chain lengths (n = 2-11), as well as with the aid of several semiempirical methods for longer chains (up to it = 21). Various energetic and structural aspects of the convergence behavior from the properties of small dications to that of an isolated, bipolaronic defect on a sufficiently long polythiophene chain are monitored and discussed. Depending on the method applied and on the structural criterion chosen, we find that an isolated, dicationic, bipolaronic defect in polythiophene extends over about 9-11 thiophene units.