Extrapolation to the Limit of a Complete Pair Natural Orbital Space in Local Coupled-Cluster Calculations

The domain-based
local pair natural orbital (PNO) coupled-cluster
DLPNO-CCSD­(T) method allows one to perform single point energy calculations
for systems with hundreds of atoms while retaining essentially the
accuracy of its canonical counterpart, with errors that are typically
smaller than 1 kcal/mol for relative energies. Crucial to the accuracy
and efficiency of the method is a proper definition of the virtual
space in which the coupled-cluster equations are solved, which is
spanned by a highly compact set of pair natural orbitals (PNOs) that
are specific for each electron pair. The dimension of the PNO space
is controlled by the TCutPNO threshold:
only PNOs with an occupation number greater than TCutPNO are included in the correlation space of a given
electron pair, whilst the remaining PNOs are discarded. To keep the
error of the method small, a conservative TCutPNO value is used in standard DLPNO-CCSD­(T) calculations. This often
leads to unnecessarily large PNO spaces, which limits the efficiency
of the method. Herein, we introduce a new computational strategy to
approach the complete PNO space limit (for a given basis set) that
consists in extrapolating the results obtained with different TCutPNO values. The method is validated on the
GMTKN55 set using canonical CCSD­(T) data as the reference. Our results
demonstrate that a simple two-point extrapolation scheme can be used
to significantly increase the efficiency and accuracy of DLPNO-CCSD­(T)
calculations, thus extending the range of applicability of the technique.