Density dependence of the roton spectrum in liquid He4

The density dependence of the roton spectrum in liquid He4 is calculated using Brillouin-Wigner perturbation theory. The trial ground-state wave function is an extended Jastrow function, including three-body factors. The three-body factors produce an improved density dependence over the pure Jastrow trial function. We note, however, that the amount of improvement depends upon the particular approximation chosen for the three-particle structure function, S(3)(k→1,k→2,k→3). Specifically, better agreement with experiment is obtained by using an approximation for S(3) which has been obtained recently in the density-phase variable theory of the weakly interacting Bose gas, when compared to the convolution approximation for S(3).