  Coupled cluster calculations are traditionally performed over Hartree-Fock
reference orbitals (HF-CC methodology). However, it has been repeatedly argued
in the literature that the use of a Kohn-Sham reference (KS-CC methodology)
might result in improved performance relative to HF-CC at the same
computational cost. In the present theoretical study, we re-examine the
relation of HF-CC and KS-CC methods by comparing the results of widely applied
truncated CC calculations (CCSD, CCSD(T), CCSDT) to the limit of full
configuration interaction (FCI), which in contrast to wave-function diagnostics
with vague physical meaning or experimental data with considerable uncertainty
serves as an undebatable reference point of accuracy. We find that apart from
incidental exceptions, the Kohn-Sham referenced CC methods show systematic
deterioration compared to HF-CC, that is, the KS-CC molecular properties
(electronic energy and density) are always farther from the FCI limit than
those obtained from HF-CC at the same coupled cluster level. Furthermore, the
introduction of common approximations (frozen core, density fitting) to the CC
calculation results in significantly higher errors in the case of KS reference.
We conclude that the use of KS reference orbitals is not expected to increase
the reliability of low-level CC energetics. Nevertheless, molecular errors from
the components of the studied chemical reaction might fortunately cancel out
resulting in illusory improvement compared to HF-CC. It is also notable that
the choice of reference orbitals has negligible influence on the results at
sufficiently high CC levels which can be estimated by test calculations or by
the magnitude of double amplitudes. Therefore, the application of KS-CC is not
unreasonable as it might bypass the difficulties of HF convergence.
