Averting the infrared catastrophe in the gold standard of quantum chemistry

Coupled-cluster theories can be used to compute electronic correlation energies of ab initio systems with systematically improvable accuracy. However, the widely-used coupled cluster singles and doubles plus erturbative triples (CCSD(T)) method is only applicable to insulating systems. 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.