Rare events in dynamic systems, like liquid phase chemical reactions, can be modeled by applying a holonomic constraint σ(q) = ξ(q) -ξ′ = 0 on a reaction coordinate ξ(q), and sampling the system at various fixed values of ξ′. For an arbitrary number of constraints the Lagrangian of the unconstrained system is extended according to Equation (q, p) = (q, p) + Σλ σ(q)