We have followed a parameterisation procedure previously used to obtain OPLS parameters for a wide variety of semiconducting polymers. In all cases, van der Waals parameters were taken directly from the OPLS force field for equivalent atom types. Atomic partial charges were obtained from quantumchemical calculations, as described in the ESI in Section S1, with the side-chains truncated to methyl groups after the tertiary carbon (i.e. R-CH 2 -CH-(CH 3 ) 2 , where R is the monomer backbone). Note that although P(NDI2OD-T2) is typically represented as having a naphthalene diimide (NDI)-bithiophene (bTh) backbone, we separated the bTh group into two thiophene (Th) groups and modelled the monomer as Th-NDI-Th, as shown in Fig. , in order to increase its symmetry, allowing for a simpler and more general parameterisation. Within the polymer, the same structure will be obtained, with the only differences being in the structure of the terminal monomers (ESI Fig. compares the two structures).