A 40 µs long MD simulation for each force field showed that except for RNA.IL.AG other force fields were unable to reproduce the slippage phenomenon. As previously observed, the RNA.IL.AG force field showed the conversion of two 2×2 GG/GG loops into three 1×1 G/G loops . During this process, fraying of terminal base pairs as well as guanosine residues flipping out from the helical axis were observed (Figure , Movie S1). The flipping-out of the guanosine residues, however, did not perturb the overall helical structure of the RNA, as they stacked back inside the helical axis. After the completion of the slippage event, the RNA construct formed three 1×1 G/G loops in anti-syn, syn-anti, and anti-syn orientations respectively. To ensure the observed orientations for three 1×1 G/G loops are stable, this simulation was extended to cover over 70 µs for the RNA.IL.AG. The MD simulations of the other three force fields were stopped after 40 µs as none of them were able to reproduce the slippage phenomenon and thus the 2×2 GG/GG → 1×1 G/G transformations (Movie S1).