A ring approximation in coupled cluster (CC) theory is worked out at
the multi reference (MR) level. The underlying CC framework is based
on generalized
normal ordering and applies the corresponding generalized Wick-theorem.
Contractions among cluster operators is avoided by adopting
a normal ordered exponential Ansatz.
The MR ring CC doubles (MR rCCD) theory is found to represent a
companion to the previously introduced extended random phase
approximation (ERPA). Equations for wavefunction
parameters are derived in parallel and compared.
Condition for the ground state consistent with ERPA
is obtained. This paves the way towards comparison with the
ERPA based correction to the
Antisymmetrized Product of Strongly
Orthogonal Geminal (APSG) reference, ERPA-APSG
