Pair Natural Orbitals for Coupled Cluster Quadratic Response Theory.

Area of Science:

• Quantum Chemistry
• Computational Chemistry

Background:

• Coupled cluster methods are essential for accurate molecular property prediction.
• Computational cost limits coupled cluster applications to large systems.
• Reduced-scaling approaches like pair natural orbitals (PNOs) improve efficiency.

Purpose of the Study:

• Extend PNOs to frequency-dependent quadratic response properties.
• Evaluate PNOs and perturbation-aware PNOs (PNO++) for response calculations.
• Develop and assess the combined-PNO++ method for simultaneous accuracy in correlation energies and response properties.

Main Methods:

• Implementation of PNOs for quadratic response properties.
• Evaluation of PNO++ methods based on approximate field-perturbed density matrices.
• Concatenation of PNO and PNO++ spaces into the combined-PNO++ method.
• Analysis of truncation errors using first electric dipole hyperpolarizability (second-harmonic generation and optical refractivity).

Main Results:

• PNOs and PNO++ show promise for calculating response properties.
• The combined-PNO++ method effectively balances accuracy for both correlation energies and response properties.
• Truncation error analysis provides insights into method performance.

Conclusions:

• The PNO family offers viable techniques for production-level coupled cluster quadratic response properties.
• Combined-PNO++ demonstrates potential for accurate and efficient large-scale quantum chemistry calculations.
• Specific PNO-based techniques are identified as most promising for future implementation.