In addition to simulating the size ratio of 0.81, we performed simulations with size ratio of 0.45 (r P = 176.5 nm and r N = 80 nm) and number ratio (positive to negative particles) of 1:2 to produce K 4 C 60 crystals (see Extended Fig. ). Simulations using experimentally realistic surface potentials of positive and negative particle, +30 mV and -50 mV, formed crystals on a surface with wall particles having a charge of -50 mV. Those shown in Extended Fig. formed with λ D =5.9 nm and volume fraction 0.0345. Bulk crystallization in Extended Fig. was observed for λ D =5.3 nm with surface charges of +/-50 mV at volume fraction 0.005. All other parameters were same as described above.