In order to cast some light onto the nature of the chemical bonding between a 8-unit oligomer of the poly(3-hexylthiophene) (P3HT) and the fullerene derivative [6,6]-phenyl-C-61-butyric acid methyl ester (PCBM) in the two stables isomers reported recently [I. Gutierrez-Gonzalez, B. Molina-Brito, A.W. Gotz, F.L. Castillo-Alvarado, J.I. Rodriguez, Chem. Phys. Lett. 612, 234 (2014)], we have performed a Bader's quantum theory of atoms in molecules (QTAIM) analysis. According to QTAIM, no covalent bonds are formed between P3HT and PCBM, and hydrogen and stacking interactions account for about 90% and 10% of the total number of bonds between P3HT and PCBM, respectively. (C) 2015 Elsevier B.V. All rights reserved.