  While the inclusion of the nonlocal correlation in fifth rung "double hybrid"
functionals is definitely beneficial, one might rightfully ask whether its
evaluation in the basis of Kohn-Sham (KS) orbitals has additional value
compared to the use of Hartree-Fock reference orbitals (in a type of multilevel
scheme). We have investigated this question for a very large and chemically
diverse dataset, GMTKN55. We conclude that KS reference orbitals are
undoubtedly beneficial, but the benefit is not as large as one might
intuitively expect.
