  We have compared the performance of Grimme style DH/DSD and Zhang-Xu-Goddard
type xDH/xDSD forms for double hybrids. In the DH and DSD forms, KS orbitals
with elevated HF exchange and damped DFT correlation are used, while in the xDH
and xDSD forms, the KS orbitals are obtained from a conventional hybrid
functional with undamped DFT correlation. Generally, the difference in
performance between DSD and xDSD functionals is small, slightly favoring xDSD.
Augmentation of the xDH form with either same-spin MP2 correlation or a
dispersion correction markedly improves performance. Best xDSD results appear
to be obtained for orbitals obtained with `exact exchange' fractions in the
50-70% range. The orbitals for xDSD appear to be fairly transferable between
different correlation functionals.
