Variational Method for Dense Systems

The variational method for calculating energy of quantum fluids, and its applications to the Bose liquid ${}_{}{}^4{}_{}{}^{}\mathrm{He}$, Fermi neutron gas, and liquid ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ are discussed. The correlation functions are parametrized by their healing distance, and can depend on the states occupied by the correlated particles in the model wave function. They are calculated by constrained variation of the lowest-order contributions. The healing distance has a prescribed value in lowest-order calculations, whereas it is sufficiently large in hopefully exact energy calculations. The many-body cluster contributions in Bose fluids are summed with successive approximations of an integral equation due to van Leeuwen et al. A simple diagrammatic cluster expansion is presented for Fermi liquids, and its direct diagrams are summed with the integral equation. The contribution of exchange diagrams is shown to decrease rapidly with the number of exchanges, and their sums are truncated after the energy has converged to within a few percent.