Reduced-Cost Second-Order Algebraic-Diagrammatic Construction Method for Core Excitations.

Area of Science:

• Computational Chemistry
• Quantum Chemistry
• Theoretical Spectroscopy

Background:

• Accurate calculation of electronic excitations is crucial in chemistry.
• Previous reduced-cost methods were limited in scope.
• Core excitations present unique computational challenges.

Purpose of the Study:

• To extend a reduced-cost computational scheme to core excitations.
• To evaluate the accuracy and efficiency of the extended method.
• To assess the computational savings for large molecular systems.

Main Methods:

• Extension of frozen virtual natural orbital and natural auxiliary function approaches.
• Application to core-valence separation (CVS) and density fitting approximations within ADC(2).
• Analysis of excitation energies and oscillator strengths for various core transitions.

Main Results:

• The method achieves significant savings in computational cost (e.g., 7-fold speedup).
• Errors in excitation energies are moderate (MAE < 0.20 eV), smaller than intrinsic errors.
• Oscillator strength errors are acceptable (MRE 0.06-0.08).
• Robustness demonstrated across different excitation types and for systems up to 100 atoms.

Conclusions:

• The developed scheme offers a practical balance between computational efficiency and accuracy for core excitations.
• Enables accurate calculations for larger systems previously intractable.
• Provides a valuable tool for theoretical spectroscopy and computational chemistry research.