When the two NPs are far from each other (r min ≥ 6.0 nm), a weaker force constant k = 600 kJ mol -1 nm -2 could be used and guarantee a proper sampling with a spacing between neighbor windows of 0.05 nm. When they are closer (r min ≤ 6.0 nm), the average force is stronger (Figure ), so the stronger force constant of 3600 kJ mol -1 nm -2 was used and the separation between the r min at neighbor windows were reduced to 0.02 nm. This change was particularly necessary to sample the region of intense attractive forces between the NPs at distances smaller than the first maximum. Even with this stronger bias potential, a few positions of the reaction coordinate displayed a bad sampling and additional simulations were performed with r min = 04.605, 04.625, 04.655, 04.675 and 04.725 nm with k = 10000 kJ mol -1 nm -2 . Although this kind of issue is rare in pmf calculations involving only soft matter, like micelles 64 and lipid bilayers, which can be performed using a single value of k across the whole reaction coordinate, when involving crystalline materials like those NPs, the sampling close to the contact distance is trickier because positions that correspond to a perfect fitting between crystallographic planes of the two