  Many recently introduced enhanced sampling techniques are based on biasing
coarse descriptors (collective variables) of a molecular system on the fly.
Sometimes the calculation of such collective variables is expensive and becomes
a bottleneck in molecular dynamics simulations. An algorithm to treat smooth
biasing forces within a multiple time step framework is here discussed. The
implementation is simple and allows a speed up when expensive collective
variables are employed. The gain can be substantial when using massively
parallel or GPU-based molecular dynamics software. Moreover, a theoretical
framework to assess the sampling accuracy is introduced, which can be used to
assess the choice of the integration time step in both single and multiple time
step biased simulations.
