Optimized pair natural orbitals for the coupled cluster methods

TL;DR

This paper introduces an optimized pair-natural orbital (PNO) approach for coupled-cluster methods that significantly reduces truncation errors and improves accuracy, especially when combined with perturbative corrections, for noncovalent binding energy calculations.

Contribution

The paper develops an iteratively-optimized PNO (iPNO) method that enhances the accuracy of coupled-cluster calculations by minimizing truncation effects and combining with perturbative corrections.

Findings

Error in CCSD energy reduced by orders of magnitude with iPNOs.

Significant improvement in noncovalent binding energy calculations, up to 100-fold accuracy increase.

Effective even at very tight PNO truncation thresholds, with minimal increase in computational cost.

Abstract

We present the coupled-cluster singles and doubles method formulated in terms of truncated pair-natural orbitals (PNO) that are optimized to minimize the effect of truncation. Compared to the standard ground-state PNO coupled-cluster approaches, in which truncated PNOs derived from first-order M{\o}ller-Plesset (MP1) amplitudes are used to compress the CC wave operator, the iteratively-optimized PNOs ("iPNOs") offer moderate improvement for small PNO ranks but rapidly increase their effectiveness for large PNO ranks. The error introduced by PNO truncation in the CCSD energy is reduced by orders of magnitude in the asymptotic regime, with an insignificant increase in PNO ranks. The effect of PNO optimization is particularly effective when combined with Neese's perturbative correction for the PNO incompleteness of the CCSD energy. The use of the perturbative correction in combination with the PNO optimization procedure seems to produce the most precise approximation to the canonical CCSD energies for small and large PNO ranks. For the standard benchmark set of noncovalent binding energies remarkable improvements with respect to standard PNO approach range from a factor of 3 with PNO truncation threshold $\tau_\text{PNO}=10^{-6}$ (with the maximum PNO truncation error in the binding energy of only 0.1 kcal/mol) to more than 2 orders of magnitude with $\tau_\text{PNO}=10^{-9}$.