tIn order to cast some light onto the nature of the chemical bonding between a 8-unit oligomer of thepoly(3-hexylthiophene) (P3HT) and the fullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester(PCBM) in the two stables isomers reported recently [I. Gutiérrez-González, B. Molina-Brito, A.W. Götz,F.L. Castillo-Alvarado, J.I. Rodríguez, Chem. Phys. Lett. 612, 234 (2014)], we have performed a Bader’squantum theory of atoms in molecules (QTAIM) analysis. According to QTAIM, no covalent bonds areformed 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.