  Hamiltonian Monte Carlo (or hybrid Monte Carlo) with partial momentum
refreshment explores the state space more slowly than it otherwise would due to
the momentum reversals which occur on proposal rejection. These cause
trajectories to double back on themselves, leading to random walk behavior on
timescales longer than the typical rejection time, and leading to slower
mixing. I present a technique by which the number of momentum reversals can be
reduced. This is accomplished by maintaining the net exchange of probability
between states with opposite momenta, but reducing the rate of exchange in both
directions such that it is 0 in one direction. An experiment illustrates these
reduced momentum flips accelerating mixing for a particular distribution.
