  The tailored coupled cluster (TCC) approach is a promising ansatz that
preserves the simplicity of single-reference coupled cluster theory, while
incorporating a multi-reference wave function through amplitudes obtained from
a preceding multi-configurational calculation. Here, we present a detailed
analysis of the TCC wave function based on model systems, which require an
accurate description of both static and dynamic correlation. We investigate the
reliability of the TCC approach with respect to the exact wave function. In
addition to the error in the electronic energy and standard coupled cluster
diagnostics, we exploit the overlap of TCC and full configuration interaction
wave functions as a quality measure. We critically review issues such as the
required size of the active space, size-consistency, symmetry breaking in the
wave function, and the dependence of TCC on the reference wave function. We
observe that possible errors caused by symmetry breaking can be mitigated by
employing the determinant with the largest weight in the active space as
reference for the TCC calculation. We find the TCC model to be promising in
calculations with active orbital spaces which include all orbitals with a large
single-orbital entropy, even if the active spaces become very large and then
may require modern active-space approaches that are not restricted to
comparatively small numbers of orbitals. Furthermore, utilizing large active
spaces can improve on the TCC wave-function approximation and reduce the
size-consistency error, because the presence of highly excited determinants
affects the accuracy of the coefficients of low-excited determinants in the
active space.
