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Computation of heats of reaction of large molecules is now feasible usingdomain-based PNO-CCSD(T) theory. However, to obtain agreement within 1~kcal/molof 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 eliminatedusing a simple two-point CPS extrapolation to the canonical limit, so thatsubsequent CBS extrapolation is not limited by residual PNO truncation error.Using the ISOL24 and MOBH35 data sets, we find that PNO truncation errors arelarger for molecules with significant static correlation, and that it isnecessary to use very tight thresholds of $T=10^{-8}$ to ensure errors do notexceed 1~kcal/mol. We present a lower-cost extrapolation scheme that usesinformation from small basis sets to estimate PNO truncation errors for largerbasis sets. In this way the canonical limit of CCSD(T) calculations on largemolecules with large basis sets can be reliably estimated in a practical way.Using this approach, we report complete basis set limit CCSD(T) reactionenergies for the full ISOL24 and MOBH35 data sets.

Improved CPS and CBS Extrapolation of PNO-CCSD(T) Energies: The MOBH35 and ISOL24 Data Sets