The self-assembly of organic molecules (octylamine) is studied by means of periodic DFT calculations including Grimme D2 dispersion scheme. It has been found that in the gas-phase dispersive forces are crucial when packing is considered, modifying considerably the potential energy surface compared to the case of a molecular dimer. In interaction with a metallic surface (platinum), the electronic interaction between adsorbate and substrate is predominant, dispersion forces mainly increasing the adsorption strength. A detailed study on the structural parameters of the molecule–metal system is reported to gain understanding on the factors affecting their stability. The intermolecular bonding in these systems is analyzed by means of the non-covalent interactions NCI index. The decomposition of the 2D packing along each direction (x, y) highlights the importance of the diagonal contributions in the stabilization of the square pattern. Moreover, the stabilizing nature of H–H interactions between neighboring molecules is supported in this model.