The retardation factor predicted by our extended jump model compares well with that effectively observed in our simulations ρ jump (Fig. ), which demonstrates the validity of the model. While our model slightly underestimates the absolute value of the retardation factor, it captures almost quantitatively (see Table ) the split in hydration dynamics between the phosphate O R and O S atoms, both in RNA and DNA, which is key for our purpose here. Note that explicitly taking into account the excluded volume coming from background potassium counterions is required in RNA to capture the full extent of the split (see Table ) and recover a better estimate of the absolute value of the retardation factor, while it has only a very minor impact in DNA. The decomposition into the different molecular factors determining the jump time (Fig. ) shows that the split in RNA between the hydration dynamics of the two phosphate oxygen atoms originates both from the larger excluded volume next to O R -coming both from the RNA and the counterions-and from the stronger initial H-bond at this oxygen.