where t represents coupled cluster amplitudes. The † â and â denotes creation and annihilation operators with respect to a Hartree-Fock vacuum in the second quantized notation and indices i, j, . . . refer to occupied orbitals, while the indices a, b, . . . to virtual ones and p, q, . . . refer the general orbitals. The coupled cluster amplitudes and energy obtained by projections to the excitation manifold, and to the reference state 0  which is generally but not necessarily a Hartree-Fock determinant, respectively.