A multireference coupled-electron pair approximation combined with complete-active space perturbation theory in local...

Area of Science:

• Quantum Chemistry
• Computational Chemistry
• Electronic Structure Theory

Background:

• Complete-Active Space Second-order Perturbation Theory (CASPT2) is widely used for multireference systems.
• CASPT2 offers computational feasibility but can lack accuracy in dynamic correlation.
• Higher-level correlation theories exist but are computationally demanding.

Purpose of the Study:

• To introduce a hybrid theory (CEPT2) combining CASPT2 and multireference Coupled-Electron Pair Approximation (CEPA).
• To improve the accuracy of electronic structure calculations for multireference systems.
• To reduce computational cost using Pair-Natural Orbitals (PNOs).

Main Methods:

• Developed a hybrid CEPT2 theory integrating CASPT2 and CEPA.
• Utilized full internal contraction to divide correlation components.
• Incorporated Pair-Natural Orbital (PNO) approximations for computational efficiency.
• Employed automatic expression and code generation for complex derivations.

Main Results:

• Demonstrated high accuracy of the CEPT2 approach through benchmark calculations.
• Assessed and quantified errors from PNO truncation.
• Showed PNO-CEPT2 energy converges to the canonical result with tighter thresholds.
• PNO-CEPT2 is more accurate than PNO-CASPT2 at identical truncation levels.

Conclusions:

• The developed CEPT2 method offers a more accurate and computationally feasible alternative for electronic structure calculations.
• PNO-CEPT2 provides a significant improvement over PNO-CASPT2 for multireference systems.
• The hybrid approach effectively balances accuracy and computational cost.