Bosonic van der Waals clusters of sizes three, four and five are studied by
diffusion quantum Monte-Carlo techniques. In particular we study the unbinding
transition, the ultra-quantum limit where the ground state ceases to exist as a
bound state. We discuss the quality of trial wave functions used in the
calculations, the critical behavior in the vicinity of the unbinding
transition, and simple improvements of the diffusion Monte Carlo algorithm.Comment: World Wide Web URL
http://www.phys.uri.edu/people/mark_meierovich/visual/Main.html contains an
informal presentation with color graphic