Metadynamics can be sped up significantly using grids, which are activated setting the GRID_MIN and GRID_MAX keywords of METAD. This makes addition of a hill to the list a bit slower (since the Gaussian has to be evaluated for many grid points) but the evaluation of the potential very fast. Since the former is usually done every few hundred steps, whereas the latter typically at every step, using grids will make the simulation faster in particular for long runs.

Notice that when restarting a simulation the history is read by default from a file and hills are added again to the grid. This allows one to change the grid boundaries upon restart. However, the first step after restart is usually very slow. Since PLUMED 2.3 you can also store the grid on a file and read it upon restart. This can be particularly useful if you perform many restarts and if your hills are large.

For the precise syntax, see METAD