Polymerization of unsubstituted indoles is studied by accurate density functional theory calculations. Relative stability of all possible dimers of indole is computed in order to understand the thermodynamics of polymerization. It is observed that 2-position is the most likely site to enhance polymerization. A selected set of trimers and tetramers which use a 2-position for linkages are generated to understand the further growth of polyindole. A study of local minima arising from different distributions of the torsional angles reveals that there are two equally probable conformations and the one with the torsional angle changing signs alternatively is slightly favored. The cyclic structures are also investigated and it is shown that it is possible to generate stable three- and four-membered cyclic structures. Finally, the structures of radical cations and intermediate states are fully optimized and the energetics of these metastable species are used to explain the competing mechanisms of radical-radical and radical-neutral pathways. (C) 2002 Published by Elsevier Science Ltd.