No abstract

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.

%e study the order of the zero temperature quantum phase transitions at density p = 1 in the onedimensional extended boson Hubbard model by examining the order parameter distribution and hysteresis effects. The phase diagram of the model at p= -', a generalization of the spin-2 XXZ Hamiltonian, is also obtained, and the possibility of a supersolid region is discussed.