This is a renewal grant for ongoing simulation studies of quantum many-body systems. Improved techniques and computing power will enable progress to be made on several more difficult and complex problems. Detailed studies will be made on the structure and phases of helium films, the structure and excitation energy of a quantized vortex, and the fluid and solid phases of helium-four very close to the melting-freezing transition. A new Fermion algorithm will be applied to study the fluid phase of helium-three and prior calculations on the momentum distribution and condensate fraction of helium-four will be improved using new sampling techniques and improved wave functions. A new study will be initiated on the ground and excited states of a helium-three impurity in liquid helium. An attempt will be made to construct a general approach to quantum mechanical soft potential problems based on hard-core modeling and a suitable perturbation approach to the remainder of the two-body potential. The formalism will be developed for the path integral method of simulating quantum systems at finite temperatures. %%% The ongoing research project will focus on the computational simulation of important problems concerning helium in its liquid and solid states. This is a difficult and very fundamental problem since quantum mechanical effects are not small for these systems. In order to deal with these computationally intensive problems, new computer techniques will be developed.
