  Computation of heats of reaction of large molecules is now feasible using
domain-based PNO-CCSD(T) theory. However, to obtain agreement within 1~kcal/mol
of experiment, it is necessary to eliminate basis set incompleteness error,
which comprises of both the AO basis set error and the PNO truncation error.
Our investigation into the convergence to the canonical limit of PNO-CCSD(T)
energies with PNO truncation threshold $T$ shows that errors follow the model
$E(T) = E + A T^{1/2}$. Therefore, PNO truncation errors can be eliminated
using a simple two-point CPS extrapolation to the canonical limit, so that
subsequent CBS extrapolation is not limited by residual PNO truncation error.
Using the ISOL24 and MOBH35 data sets, we find that PNO truncation errors are
larger for molecules with significant static correlation, and that it is
necessary to use very tight thresholds of $T=10^{-8}$ to ensure errors do not
exceed 1~kcal/mol. We present a lower-cost extrapolation scheme that uses
information from small basis sets to estimate PNO truncation errors for larger
basis sets. In this way the canonical limit of CCSD(T) calculations on large
molecules with large basis sets can be reliably estimated in a practical way.
Using this approach, we report complete basis set limit CCSD(T) reaction
energies for the full ISOL24 and MOBH35 data sets.
