The Gaussian widths used are 0.2 Å and 0.05 for the nitrogen distance and coordination number respectively. The Gaussians added to form the history-dependent potential had an initial height of 1.0 kJ/mol, at 120 ps the height was increased to 2.5 kJ/mol, then a final increase at 140 ps to 5.0 kJ/mol, which was then damped throughout the simulation. At convergence, the relative free energy can be calculated from the applied biases: 25