  We consider the performance of combined PNO-F12 approaches for the
dissociation energy of water clusters as large as (H2O)20 by comparison to
canonical CCSD(T)/CBS reference values obtained through n-body decomposition of
post-MP2 corrections. We find that PNO-LCCSD(T)-F12b approaches with "Tight"
cutoffs are generally capable of reproducing canonical CCSD(T) interaction
energies to within ~0.25% and isomerization energies to ~1.5%, while requiring
only a fraction of the canonical computational cost. However, basis set
convergence patterns and effect of counterpoise corrections are more erratic
than for canonical calculations, highlighting the need for canonical benchmarks
on closely related systems.
