Predicting Energetic Disorder: A quantum chemical method is used to calculate the LUMO energies of all possible isomers of the bis and tris adducts of the fullerene, [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). The calculated energy level distributions agree well with the observed mean and spread of LUMO energies as determined using solution differential pulse voltammetry (DPV). We propose this method as a powerful tool for the design and functional optimisation of novel fullerenes, as well as other classes of pi-conjugated molecules with multiple isomers. [GRAPHICS] .