  Coupled-cluster theories can be used to compute ab initio electronic
correlation energies of real materials with systematically improvable accuracy.
However, the widely-used coupled cluster singles and doubles plus perturbative
triples (CCSD(T)) method is only applicable to insulating materials. For
zero-gap materials the truncation of the underlying many-body perturbation
expansion leads to an infrared catastrophe. Here, we present a novel
perturbative triples formalism that yields convergent correlation energies in
metallic systems. Furthermore, the computed correlation energies for the three
dimensional uniform electron gas at metallic densities are in good agreement
with quantum Monte Carlo results. At the same time the newly proposed method
retains all desirable properties of CCSD(T) such as its accuracy for insulating
systems as well as its low computational cost compared to a full inclusion of
the triples. This paves the way for ab initio calculations of real metals with
chemical accuracy.
