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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 tai to zero. In this sense MP2 can be derived as a simplification of CC2. But it should be noted that CC2 energies and geometries are usually not more accurate than MP2.

For CCS and CIS the double-substitution amplitudes are excluded from the cluster expansion and the single-substitution amplitudes for the ground state wavefunction are zero for closed-shell RHF and open-shell UHF reference wavefunctions and thus energy is identical to the SCF energy.

For the Methods CIS(D), CIS(D ) 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.



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