where Ψ 0 is the reference Hartree-Fock wave function given by a single Slater determinant, T is the electron excitation operator, and H is the molecular Hamiltonian, within the Born-Oppenheimer approximation. With the T operator, any number of electron excitations can be considered, up to the number of electrons in the system. More excitations considered means a larger runtime, in exchange for greater accuracy. Coupled-cluster methods are defined by the highest level of electronic excitations that are considered. For the CCSD method, T = T 1 + T 2 , such that