On the solid- and liquidlike nature of quantum clusters in their ground state

The ground state of pristine clusters of (paraH(2))(N) and (orthoD(2))(N) of size ranging from N=11 to 55 is examined by means of the variational path integral method. The chemical potential is calculated for two different interaction models and it is shown that the location of magic numbers indeed depends on the chosen interaction potential. Density profiles are calculated and reveal the difference between the two isotopes with regards to shell structure. The magnitude of relative pair distance and position fluctuations is used to asses the rigidity of these finite-size quantum systems. A comparison of generic and specific distance fluctuations as a function of cluster size is proposed as a probe of the appearance of rigidity in the clusters. It is found that smaller (paraH(2))(N) clusters are fluidlike and start to display increased rigidity for clusters of size N> or =26, whereas (orthoD(2))(N) clusters of N=13 and N> or =19 are rigid. Small clusters exhibit structures loosely based on an anti-Mackay icosahedral motif. An anti-Mackay to Mackay transition at N=41-42 is suggested.