Structure and collective excitations of He4 clusters

We compute zero-temperature ground-state energies, one- and two-body densities, collective-excitation spectra, transition densities, and static and dynamic structure functions of He4 clusters up to a cluster size of N=112 particles. The ground-state properties are computed using a second-order diffusion Monte Carlo algorithm with Jastrow and triplet trial functions used for importance sampling. Excitation energies, transition densities, and dynamic structure functions are obtained by solving a generalized Feynman eigenvalue equation. We determine the systematic variation of collective energies with cluster size, demonstrate the existence of persistent oscillations in transition densities, evaluate the strength of collective modes quantitatively, and show how the cluster continuum excitation spectrum can be directly mapped by the dynamic structure function. By comparison with the full static structure function, the collective quadrupole state is found to exhaust approximately 25% of the total strength.