We present a computationally efficient molecular dynamics method, based on holonomic constraints, devised to estimate the rate constants of rare activated events of short duration. We assume that the process is described by a reaction coordinate xi (r), a well-defined function in configuration space, and we constrain the system at the bottleneck region by prescribing the value of xi (r) MD trajectories sample phase space according to a biased configurational distribution and averages can be taken from such a new Blue Moon. ensemble, with suitable reweighting, to study rare events. (C) 2000 Elsevier Science B.V. All rights reserved.