  Metadynamics is a commonly used and successful enhanced sampling method. By
the introduction of a history dependent bias which depends on a restricted
number of collective variables(CVs) it can explore complex free energy surfaces
characterized by several metastable states separated by large free energy
barriers. Here we extend its scope by introducing a simple yet powerful method
for calculating the rates of transition between different metastable states.
The method does not rely on a previous knowledge of the transition states or
reaction co-ordinates, as long as CVs are known that can distinguish between
the various stable minima in free energy space. We demonstrate that our method
recovers the correct escape rates out of these stable states and also preserves
the correct sequence of state-to-state transitions, with minimal extra
computational effort needed over ordinary metadynamics. We apply the formalism
to three different problems and in each case find excellent agreement with the
results of long unbiased molecular dynamics runs.
