PNO++: Perturbed Pair Natural Orbitals for Coupled Cluster Linear Response Theory

Area of Science:

• Computational chemistry
• Quantum chemistry
• Theoretical chemistry

Background:

• Accurate calculation of molecular properties is crucial in chemistry.
• Coupled cluster linear response (CCLR) methods offer high accuracy but are computationally expensive for large molecules.
• Reduced-scaling methods are needed to make CCLR methods tractable for larger systems.

Purpose of the Study:

• To investigate the perturbed pair natural orbital (PNO++) approach for calculating molecular properties.
• To develop and analyze a new "combined PNO++" method for improved accuracy and efficiency.
• To assess the performance of PNO++ and combined PNO++ for correlation energies and linear response properties.

Main Methods:

• Examined the PNO++ approach using a coupled cluster singles and doubles (CCSD) reference.
• Analyzed truncation errors in correlation energies, dynamic polarizabilities, and specific rotations.
• Introduced and evaluated the "combined PNO++" method by incorporating PNO space orbitals into PNO++.

Main Results:

• The PNO++ method optimizes orbital space for response properties using a field-perturbed density matrix.
• The combined PNO++ approach recovers CCSD correlation energy accuracy.
• Combined PNO++ preserves the favorable convergence behavior of PNO++ for linear response properties.

Conclusions:

• The combined PNO++ method offers a promising reduced-scaling approach for accurate molecular property calculations.
• This method balances computational efficiency with the accuracy of traditional coupled cluster methods.
• The combined PNO++ approach is systematically improvable and suitable for large molecules.