  While restricted single-reference coupled cluster theory truncated to singles
and doubles (CCSD) provides very accurate results for weakly correlated
systems, it usually fails in the presence of static or strong correlation. This
failure is generally attributed to the qualitative breakdown of the reference,
and can accordingly be corrected by using a multi-determinant reference,
including higher-body cluster operators in the ansatz, or allowing symmetry
breaking in the reference. None of these solutions are ideal; multi-reference
coupled cluster is not black box, including higher-body cluster operators is
computationally demanding, and allowing symmetry breaking leads to the loss of
good quantum numbers. It has long been recognized that quasidegeneracies can
instead be treated by modifying the coupled cluster ansatz. The recently
introduced pair coupled cluster doubles (pCCD) approach is one such example
which avoids catastrophic failures and accurately models strong correlations in
a symmetry-adapted framework. Here we generalize pCCD to a singlet-paired
coupled cluster model (CCD0) intermediate between coupled cluster doubles and
pCCD, yielding a method that possesses the invariances of the former and much
of the stability of the latter. Moreover, CCD0 retains the full structure of
coupled cluster theory, including a fermionic wave function, antisymmetric
cluster amplitudes, and well-defined response equations and density matrices.
