Ground-State calculations for other methods than CC2:

The MP2 equations and the energy are obtained by restricting in the CC2 equations the single-substitution amplitudes $t_{ai}$ mathend000# to zero. For CCS/CIS the double-substitution amplitudes are excluded from the cluster expansion and in this case the single-substitution amplitudes for the ground state become zero and the energy is identical to the SCF energy. For the Methods CIS(D), CIS(D$_\infty$ mathend000#) and ADC(2) the ground state is identified with the MP2 ground state.