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

Computation of heats of reaction of large molecules is
now feasible
using the domain-based pair natural orbital (PNO)-coupled-cluster
singles, doubles, and perturbative triples [CCSD(T)] theory. However,
to obtain agreement within 1 kcal/mol of experiment, it is necessary
to eliminate basis set incompleteness error, which comprises 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 the PNO truncation threshold T shows that errors follow the model E(T)=E+AT1/2. 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 the 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 that errors do not exceed 1 kcal/mol.
We present a lower-cost extrapolation scheme that uses information
from small basis sets to estimate the PNO truncation errors for larger
basis sets. In this way, the canonical limit of CCSD(T) calculations
on sizable molecules with large basis sets can be reliably estimated
in a practical way. Using this approach, we report near complete
basis set (CBS)-CCSD(T) reaction energies for the full ISOL24 and
MOBH35 data sets.