  We propose a new method to obtain kinetic properties of infrequent events
from molecular dynamics simulation. The procedure employs a recently introduced
variational approach [Valsson and Parrinello, Phys. Rev. Lett. 113, 090601
(2014)] to construct a bias potential as a function of several collective
variables that is designed to flood only the associated free energy surface up
to a predefined level. The resulting bias potential effectively accelerates
transitions between metastable free energy minima while ensuring bias-free
transition states, thus allowing accurate kinetic rates to be obtained. We test
the method on a few illustrative systems for which we obtain an order of
magnitude improvement in efficiency relative to previous approaches, and
several orders of magnitude relative to unbiased molecular dynamics. We expect
an even larger improvement in more complex systems. This and the ability of the
variational approach to deal efficiently with a large number of collective
variables will greatly enhance the scope of these calculations. This work is a
vindication of the potential that the variational principle has if applied in
innovative ways
