All implementations of HMC use numerical integrators requiring a step size (equivalently, discretization time interval).
If step_size
is too large, the leapfrog integrator will be
inaccurate and too many proposals will be rejected. If step_size
is too small, too many small steps will be taken by the leapfrog integrator
leading to long simulation times per interval. Thus the goal is to balance the
acceptance rate between these extremes.