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The MP2 equations and the energy are obtained by restricting in the
CC2 equations the single-substitution amplitudes 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) 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.