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18 p3ht - poly(3-hexylthiophene)carbon atoms. Its regioregular form is the most used variantof the polymer in optolectronic applications [85].Figure 2.1.: P3HT molecule composed by 16 thiophenes.The first step in the study of P3HT is to validate themodel interaction. Specifically, we focus on the π − π interactionthat is dominated by long-range van der Waals dispersiveforces and we adopt as a test case a pair of simplethiophene rings for which accurate first-principles calculationsbeyond Hartree-Fock theory are available (CCSD(T)and MP2 [86]). In Figure 2.2 the calculated MPMD results(symbols) are reported together with ab initio results (continousand dotted lines for CCSD(T) and MP2, respectively).Our model potential reproduce quite well the first-principlesresults being in between the CCSD(T) and MP2 curves, inparticular for the dispersive R −6 tail and for the estimationof the minimum energies [3].We generate two P3HT molecules each formed by 8 monomers(16 thiophenes) with periodic boundary conditions alongthe backbone direction. The equilibrium lattice parameteralong the backbone is 7.75 Å [3].The assembling of the P3HT chains can be driven bytwo main contributions: the π − π interaction between thearomatic rings of the backbones of neighboring molecules,promoting the parallel stacking of different chains, and thechain interdigitation, inducing molecules alignment in thesame plane [3] (see Figure 2.3).As for the π − π binding energy, we consider two polymerchains at varying distance in a face-to-face configurationand we found a minimum at 4 Å (see Figure 2.4).The interactions between backbones are dominated by thethiophene-thiophene interactions calculated above. Additionalsmaller dispersive and electrostatic contributions due