Unless explicitly stated in the figure caption, properties were averaged over an (N, V, T ) trajectory of 100 ps for MLP simulations. Frames were taken every 100 fs for the radial distribution function (RDF), bond angle distribution and coordination number, every 500 fs for the mean square displacement (MSD), every 1 ps for ring statistics and every 10 ps for pore statistics. Reference ab initio properties were computed as in Ref. 15.