The solid substrate was modelled as a single layer of either atomistic graphene or silicon, positioned with atoms centred at z = 0. These substrates have been shown experimentally to interact strongly (graphene) or weakly (silicon) with OSCs and to give rise to face-on and side-on orientations, respectively, of the common OSC polymer poly(3-hexylthiophene) (P3HT). Parameters (non-bonded interactions and bond length) for graphene were taken from the OPLS-AA force field for aromatic carbons (ε LJ = 0.039ε, σ LJ = 1.11σ , bond length = 0.44σ , for ε = 1.8 kcal/mol and σ = 3.2 Å) and atoms positioned in the hexagonal lattice of graphene with separation taken from the OPLS-AA force field as the bond length for aromatic carbons. OPLS parameters were also used for silicon (ε LJ = 0.056ε, σ LJ = 1.70σ ) and atoms positioned to give the fcc(001) plane of silicon's diamond cubic lat-