Computational reinvestigation of the bithiophene torsion potential q Guido Raos * , Antonino Famulari, Valentina Marcon Dipartimento di Chimica, Materiali e Ingegneria Chimica ‘‘G. Natta’’, Politecnico di Milano, Via L. Mancinelli 7, 20131 Milan, Italy Received 26 February 2003; in final form 25 August 2003 Published online: 16 September 2003 Abstract We present the results of ab initio calculations of the torsion potential of 2,2 0 -bithiophene, using the MP2, CCSD, CCSD(T) and B3LYP electron correlation methods. Special emphasis is given to the systematic investigation of basis set effects by the use of correlation-consistent bases. We find that the MP2 and coupled-cluster potential energy curves agree well with each other, for a given basis set. However, unlike B3LYP, they are rather sensitive to the quality of the basis. The previously observed disagreement between the B3LYP and MP2 methods, especially on the degree of ring coplanarity in the minimum-energy cis and trans conformations, is considerably reduced upon adoption of a large and flexible basis set for the latter. Ó 2003 Elsevier B.V. All rights reserved. 1. Introduction Oligo- and polythiophenes are among the most promising and widely studied materials for organic electronic and optoelectronic applications [1–4]. Their aggregation and self-assembly properties are crucial for the charge mobility in organic thin film transistors or for the quantum yield of the lumi- nescence in light-emitting diodes, to name two important examples [2,3]. As a first step in our effort to study many-chain interactions in oli- gothiophenes by molecular dynamics simulations, we have re-examined the problem of the torsion potential in the dimer (2,2 0 -bithiophene or 2,2 0 - bithienyl), which is depicted below in its trans- planar (also known as anti-planar) and cis-planar (or syn-planar) conformations. The main features of the torsion energy profile of bithiophene have been known for at least a decade [5–13]. The absolute minimum has a trans- distorted (TD, also known as anti-gauche) con- formation, with an S–C–C–S torsion angle / ffi150°. There is a second pair of minima Chemical Physics Letters 379 (2003) 364–372 www.elsevier.com/locate/cplett q Dedicated to Prof. Giuseppe Allegra, on occasion of his 70th birthday. * Corresponding author. Fax: +39-02-23993080. E-mail address: guido.raos@polimi.it (G. Raos). 0009-2614/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2003.08.060