Chain Length Effects on Nonlinear Excitation Transitions in Trans- Polyacetylene PEDRO HENRIQUE de OLIVEIRA NETO, WILIAM FERREIRA da CUNHA, RICARDO GARGANO, GERALDO MAGELA e SILVA Institute of Physics, University of Brası ´lia, Brası ´lia, DF, CEP 70919-970, Brazil Received 17 December 2007; accepted 29 January 2008 Published online 20 June 2008 in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/qua.21649 ABSTRACT: We investigate the nonlinear excitation transitions in a trans- polyacetylene chain. The Su–Schrieffer–Heeger model was used. The chain length was successively changed so that this modification effects could be explored. The system is composed of a polyacetylene chain initially with a charged polaron. An electron ionization is then simulated and the formation of a couple of solitons took place. It was observed that the chain length influences the spectra, raising concerns about the proper chain length to simulate very long chains. Different boundary conditions were also investigated, and as a result we found that steady chain edges are more appropriate to model long chains, when transitions take place. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem 108: 2507–2511, 2008 Key words: chain length; nonlinear excitation; transitions 1. Introduction C onjugated polymers, which are those with al- ternating simple and double bonds in the main chain, may show high electrical conductivity [1, 2] under certain conditions. There are great numbers of conducting plastic applications, vary- ing from laser to diodes and transistors. Trans- polyacetylene was the first conducting polymer to be built, and besides having the wanted properties of a conducting plastic, it has a very simple struc- ture and has a low production price. Different from the usual conductors, the charge carriers in these materials are chain defects caused by a nonlinear response of the electron-lattice cou- pling [3, 4]. These responses are excitations that cause a distortion in the chain and have the prop- erty of free movement through the chain, and are known as polarons, bipolarons, and solitons. Pol- arons and bipolarons are quasi-particles character- Correspondence to: P. H. de Oliveira Neto; e-mail: pedrohenrique@fis.unb.br International Journal of Quantum Chemistry, Vol 108, 2507–2511 (2008) © 2008 Wiley Periodicals, Inc.