Above, |Ψ 0 ⟩ represents the reference determinant computed from a preceding HF computation. T represents the electron excitation operator in CC theory, and H is the molecular Hamiltonian within the Born-Oppenheimer approximation. With the electronic excitation operator, any number of electron excitations may be considered, up to every single active electron in the molecule. The more excitations considered, the closer the computed correlation energy is to the FCI limit. The cost of a coupled-cluster computation scales as O(n 2r+2 ) with respect to maximum excitation order r. For kcal mol -1 accuracy (chemical accuracy) in chemical applications, at least triples excitations need to be considered. Unfortunately, the cost of treating triples exactly scales as O(n ). This has led to the development of the popular CCSD(T) method, 9 with the triples correction energy derived by adding back fourth-order and fifth-order terms obtained from many-body perturbation theory. 10