Which model density is best in pair natural orbital local correlation theory?

TL;DR

This paper evaluates the impact of model density choices in pair natural orbital local correlation theory, finding second-order perturbation theory offers optimal balance between computational cost and accuracy, especially when orbital coupling is considered.

Contribution

It systematically assesses the effect of different model densities in PNO-CCSD(T), highlighting the importance of orbital coupling and identifying the best compromise for accuracy and efficiency.

Findings

Second-order perturbation theory yields the best accuracy-cost balance.

Coupling between localised occupied orbitals is crucial for low errors.

Remaining errors mainly stem from (T) energy treatment.

Abstract

Low-scaling electron correlation theory based on the pair natural orbital approximation, PNO-CCSD(T), has become a powerful computational tool. Motivated by the recent discovery of large errors for organometallic molecules, we assess the role of the model density used to discard unimportant contributions. We find that second-order perturbation theory provides the best compromise between cost and accuracy, but coupling between localised occupied orbitals must be accounted for. Errors in the CCSD energy are then well below 1~kcal/mol, even for molecules with moderate multi-reference character, and the primary remaining source of errors lies in the treatment of the (T) energy contribution.