In this Letter, a case is reported where a small basis set superposition error (BSSE) dependence on the geometry leads to a large change in the optimized structure. The rotational potential energy curves of the p-difluorobenzene-N-2 (p-DFB-N-2) complex were calculated with MP2, MP3, MP4(SDQ) and basis sets at different levels. For all calculations with the BSSE correction, similar rotational potential curves and the same minimum point were obtained. On the other hand, the uncorrected potential curves were quite different from the above ones and from each other. (C) 2002 Elsevier Science B.V. All rights reserved.