∆ i is the RMSD between the current structure and the structure saved after i ps of the simulation. In this way the meta-MD simulation is "pushed" towards unexplored molecular configurations e.g. different conformations and chemical reactions. The magnitude of this pushing is controlled by the parameter k push . The width of these Gaussian biasing potentials is controlled by the parameter α. In previous studies we have found the combination of k push = 0.05 E h and α = 0.3 Bohr -2 to generally work well for finding low-barrier reactions. The molecular system is encapsulated in a spherical cavity for which a scaling parameter s is used to control the size. We have previously found it necessary to gradually decrease this cavity for bimolecular reactions. This is done by decreasing s with 0.02 every 5 ps in case no reaction has occurred. We use s = 1.0 as the initial scaling factor for the cavity.