Coupled cluster theory for condensed phase spectroscopy

We use state-of-the-art equation-of-motion coupled-cluster theory with single and double excitations (EOM-CCSD) to calculate the dynamic structure factor of the uniform electron gas. Our calculations are performed at densities corresponding to the valence electron densities of common metals. We compare our results to those obtained using the random-phase approximation, which is known to provide a reasonable description of the collective plasmon excitation. We find that EOM-CCSD, instead of providing a perturbative improvement on the RPA plasmon, predicts a many-state plasmon resonance, where each contributing state has a double-excitation character of 80 percent or more. This finding amounts to an ab initio treatment of the plasmon linewidth and highlights the strongly correlated nature of lifetime effects in condensed-phase electronic structure theory.