We study the assembling of P3HT chains in vacuo by means of a combination of first principles density functional theory and model potential molecular dynamics. We find that, in the absence of any external constraints, the pi-pi interchain interaction between thiophenes is the major driving force for the assembling. Single chains stack in a staggered geometry giving rise to the formation of two-dimensional hydrophobic foils. These, in turn, assemble into a zigzag bulk polymer structure in agreement with experimental findings. Finally, in the presence of some external constraint (like e.g. a substrate), when the alignment of single chains is favored instead of the stacking, a different bulk structure is possible where thiophene rings are aligned.