  Low-scaling electron correlation theory based on the pair natural orbital
approximation, PNO-CCSD(T), has become a powerful computational tool. Motivated
by the recent discovery of large errors for organometallic molecules, we assess
the role of the model density used to discard unimportant contributions. We
find that second-order perturbation theory provides the best compromise between
cost and accuracy, but coupling between localised occupied orbitals must be
accounted for. Errors in the CCSD energy are then well below 1~kcal/mol, even
for molecules with moderate multi-reference character, and the primary
remaining source of errors lies in the treatment of the (T) energy
contribution.
