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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}$ 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$) and ADC(2) the ground state is identified with the MP2 ground state to define is total energy of the excited state, which is needed for the definition of gradients and (relaxed) first-order properties which are obtained as (analytic) derivatives the total energy.