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The CC2 ground-state energy
is--similarly to other coupled-cluster energies--obtained
from the expression

where the cluster operator is expanded as
with

(for a closed-shell case; in an open-shell case an additional spin
summation has to be included).
The cluster amplitudes and are obtained as solution
of the CC2 cluster equations [82]:

with
The residual of the cluster equations
is the
so-called vector function.
The recommended reference for the CC2 model is ref. [82],
the implementation with the resolution-of-the-identity approximation, RI-CC2,
was first described in ref. [10].

**Subsections**

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** Up:** Second-Order Approximate Coupled-Cluster (CC2)
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