This means that CCSD and CCSD(T) methods are intractable for systems with more than around 30 atoms on a typical workstation. Therefore, it is useful to devise a series of approximations to CCSD and CCSD(T) that allow them to be useful for larger molecular complexes, such as pharmaceutical molecules, protein fragments, and smaller whole proteins (like crambin and insulin) to allow for increased applicability of high-accuracy quantum chemistry to fields like drug discovery and computational biology. Currently, cheaper methods like density functional theory (DFT) or Møller-Plesset perturbation theory (MP2) are applied to these problems, but they do not have the accuracy of coupled-cluster.