Abstract
ABSTRACTA ring approximation in coupled-cluster (CC) theory is worked out at the multireference (MR) level. The underlying CC framework is based on generalised normal ordering and applies the corresponding generalised Wick's theorem. Contractions among cluster operators are 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 antisymmetrised product of strongly orthogonal geminal (APSG) reference, ERPA-APSG.
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