Dynamical correlation energy of metals in large basis sets from downfolding and composite approaches

Coupled-cluster theory with single and double excitations (CCSD) is a promising ab initio method for the electronic structure of three-dimensional metals, which second-order perturbation (MP2) diverges in thermodynamic limit. However, due to high cost poor convergence CCSD respect basis size, applying periodic systems often leads large set errors. In common "composite" method, MP2 used recover missing dynamical correlation energy through focal-point correction, but inadequacy metals raises questions about this approach. Here we describe how high-energy treated by can be "downfolded" into low-energy active space CCSD. Comparing composite downfolding approaches perform uniform electron gas, find that latter converges more quickly size. Nonetheless, approach surprisingly accurate because it removes problematic treatment near Fermi surface. Using estimate combined complete limits, recovers over 90% exact at $r_s=4$. We also test random-phase approximation place MP2, yielding effective expensive.