Scalable Electron Correlation Methods. 3. Efficient and Accurate Parallel Local Coupled Cluster with Pair Natural...

Area of Science:

• Computational Chemistry
• Quantum Chemistry
• Electronic Structure Theory

Background:

• Accurate electronic structure calculations are crucial for understanding molecular properties and reactions.
• Coupled-cluster methods, particularly singles and doubles (CCSD), are highly accurate but computationally expensive for large systems.
• Local approximations and pair natural virtual orbitals (PNOs) have been developed to reduce the computational cost of CCSD.

Purpose of the Study:

• To present a well-parallelized local singles and doubles coupled-cluster (LCCSD) method utilizing pair natural virtual orbitals (PNOs).
• To improve the accuracy of LCCSD calculations for large molecular systems, particularly those dominated by long-range dispersion interactions.
• To investigate the impact of various approximations on energy calculations and establish criteria for reliable results.

Main Methods:

• Construction of PNOs using projected atomic orbitals (PAOs) and orbital specific virtual orbitals (OSVs).
• Introduction of a hierarchy for close, weak, and distant electron pairs based on local Møller-Plesset perturbation theory (LMP2) energies.
• Treatment of close and weak pairs by higher-order LCCSD approximations, distant pairs by spin-component scaled LMP2 (SCS-LMP2) with multipole approximations, and introduction of projection approximations for efficiency.

Main Results:

• The developed LCCSD-PNO method achieves high accuracy for large systems, with errors in relative energies below 1 kJ mol⁻¹ when appropriate PNO domain sizes and energy criteria are used.
• The method exhibits near-linear scaling for one-dimensional systems, and efficient parallelization allows accurate calculations for 3D molecules with ~100 atoms in hours.
• Errors due to approximations are minimized with sufficiently large PNO domains, though basis set superposition errors can arise with very large domains.

Conclusions:

• The presented LCCSD-PNO method offers a significant advancement in accurate and efficient quantum chemical calculations for large molecular systems.
• The hierarchical treatment of electron pairs and judicious use of approximations, guided by an energy criterion for PNO domain selection, are key to its success.
• The method provides a robust framework for studying complex chemical phenomena, with explicitly correlated variants (e.g., PNO-LCCSD-F12) addressing remaining basis set limitations.