We present the results of ab initio calculations of the torsion potential of 2,2'-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. (C) 2003 Elsevier B.V. All rights reserved.