Local complete active space second-order perturbation theory using pair natural orbitals (PNO-CASPT2)

Area of Science:

• Computational Chemistry
• Quantum Chemistry
• Theoretical Chemistry

Background:

• Accurate calculation of molecular properties is crucial in chemistry.
• Traditional methods often scale poorly with molecular size, limiting their application.
• Local approximations offer a path to more efficient computations.

Purpose of the Study:

• To develop a linear-scaling CASPT2 (Complete Active Space second-order perturbation theory) method.
• To enable efficient computation of excitation and correlation energies for larger systems.
• To validate the accuracy of local approximations in CASPT2 calculations.

Main Methods:

• Employs local approximations for linear scaling of computational effort.
• Utilizes localized inactive orbitals and pair-natural orbitals (PNOs) for virtual spaces.
• Incorporates multipole approximations for distant pair energies and local density fitting for integral evaluation.

Main Results:

• The developed CASPT2 method achieves linear scaling with molecular size.
• Errors introduced by local approximations are demonstrated to be minimal.
• The method's efficiency and accuracy are validated for excitation and correlation energies.

Conclusions:

• The presented linear-scaling CASPT2 method provides an efficient and accurate approach for electronic structure calculations.
• Local approximations are well-controlled and introduce negligible errors.
• This method significantly advances the computational feasibility for studying larger molecular systems.