  An algorithm for separating the high- and low-frequency molecular dynamics
modes in Hybrid Monte Carlo simulations of gauge theories with dynamical
fermions is presented. The separation is based on splitting the pseudo-fermion
action into two parts, as was initially proposed by Hasenbusch. We propose to
introduce different evolution time-scales for each part. We test our proposal
in realistic simulations of two-flavor O(a) improved Wilson fermions. A
speed-up of more than a factor of three compared to the standard HMC algorithm
is observed in a typical run.
