A density functional theory (DFT) study of the supramolecular dimer formed by a 8-unit oligomer of the poly(3-hexylthiophene) (P3HT) and the fullerene derivative [6,6]-phenyl-C-61-butyric acid methyl ester (PCBM) is presented. A dispersion-corrected exchange-correlation potential was used to afford for the long-range van der Waals interactions. Our calculations predict two stable isomers in which the P3HT 8-mer forms a U-shaped structure surrounding the PCBM. From time-dependent DFT calculations it was determined that the maximum absorption intensity of the P3HT-PCBM dimer undergoes a blue-shift of about 80nm with respect to the isolated P3HT oligomer in qualitative agreement with the experimental facts. (C) 2014 Elsevier B.V. All rights reserved.