Most bonded parameters for bond lengths, angles, and dihedrals were taken directly from the OPLS force field for equivalent atom types, while the equilibrium bond lengths and angles were obtained from the quantum-chemistry optimised geometry of the monomer. The exceptions were the bond stretching po- Fig. Chemical structure of the P(NDI2OD-T2) monomer with the CG model overlaid. CG sites are labelled (17) and coloured by their site type. To preserve the backbone geometry of the AA representation, two dierent site types for the thiophenes (1 and 7) and imides (2 and 6) were dened, which have the same non-bonded and bond length parameters, but dierent bond angle parameters. Note that the terminal methyl group of one of the side-chains was not included in the coarse-graining in order to reduce the number of site types and to facilitate parameterisation.