Among the earliest method that approximates full quadruples in coupled-cluster theory is the the [Q] approach of Kucharski, Bartlett, and co-workers. In the [Q] approach, the energy is obtained from the quadruples terms present through fifth order in many body perturbation theory, and consists of the coupling of doubles amplitudes with the quadruples amplitude, as well as the fluctuation potential W , defined through the partitioning of the electronic Hamiltonian H = F + W , where F is the Fock operator.