where the operators, a † , b † , i, j are the fermionic creation and annihilation operators adding or removing electrons to molecular orbitals ϕ a , ϕ b , ϕ i , ϕ j . The choice of reference Φ 0 determines the separation between the occupied and virtual spaces , The amplitudes t are found by solving the coupled-cluster equations obtained by projecting the similarity transformed Schrödinger equation