Triplet correlations in the quantum hard-sphere fluid.

A study of three-particle correlations in the quantum hard-sphere fluid far from exchange is presented. The three types of triplet correlations in a monatomic quantum fluid (instantaneous, linear response, and centroids) are analyzed by utilizing (a) the density derivatives of the corresponding quantum pair radial correlation functions, (b) closures for triplet functions, and (c) path-integral Monte Carlo (PIMC) simulations that have concentrated on the fixing of equilateral and isosceles correlations. For the sake of comparison, the classical hard-sphere fluid is also studied with tools (a) and (b) and Monte Carlo (MC) simulations. The relative usefulness of density derivatives combined with closures is discussed in light of the PIMC and MC results. The exact PIMC correlations between quantum triplets show features that resemble those known to occur at the pair level, such as the close proximity between the instantaneous and the three-particle linear response, the much more pronounced features in centroid triplet structures, and the same global patterns with changes in density and temperature such as the outward shifts of the structures with decreasing temperature and density.