  We present an algorithm for rigid body diffusion Monte Carlo with importance
sampling, which is based on a rigorous short-time expansion of the Green's
function for rotational motion in three dimensions. We show that this
short-time approximation provides correct sampling of the angular degrees of
freedom, and provides a general way to incorporate importance sampling for all
degrees of freedom. The full importance sampling algorithm significantly
improves both calculational efficiency and accuracy of ground state properties,
and allows rotational and bending excitations in molecular van der Waals
clusters to be studied directly.
