The 5' and 3' ends polyribonucleotides were modeled as hydroxyl groups, without phosphate. All systems were prepared and solvated using the CHARMM-GUI server with the CHARMM36m force field parameters in combination with the CHARMM version of the TIP3P water model, which was kept rigid in all simulations using the SETTLE constraint algorithm. All simulations were performed using Gromacs 2020.1 and the replicas and mass scaling were controlled through asyncmd. Two 10 ns-long equilibration runs were performed, first in the NVT ensemble at constant volume and temperature and then in the NPT ensemble at constant pressure and temperature. The following production simulations in the NPT ensemble were run for times indicated in Supporting Table . All simulations were performed using a 2 fs time step at T = 310 K and p = 1 bar with the stochastic velocity rescaling thermostat 24 (τ t = 1 ps) and the Parrinello-Rahman barostat (τ p = 5.0 ps).