FR-TO ΔSCF: A Robust and Systematic Framework for Core Excitations.

Area of Science:

• Computational Chemistry
• Quantum Chemistry
• Spectroscopy

Background:

• Core-level excited states are crucial for understanding chemical processes.
• Accurate calculation of these states is computationally demanding.
• Existing methods face challenges with orbital relaxation and convergence.

Purpose of the Study:

• To present a robust ΔSCF scheme for core-level excited states.
• To improve the accuracy and reliability of excitation energy calculations.
• To offer a computationally efficient alternative to existing methods.

Main Methods:

• A freeze-and-release (FR) optimization strategy using transition orbitals (TOs).
• Constraining TOs from subspace projected Tamm-Dancoff approximation (TDA) calculations.
• A two-stage process: freezing TOs for relaxation, then releasing them for final optimization.

Main Results:

• Improved excitation energies compared to iterative vector interaction TDA (iVI TDA).
• Computational cost comparable to ground-state SCF calculations.
• More reliable convergence than conventional maximum overlap method-type (MOM-type) schemes.

Conclusions:

• The FR-TO procedure provides a physically motivated and systematic framework for core-level ΔSCF.
• It enhances accuracy and reliability, especially for complex systems.
• Offers a cost-effective approach for core-level excited state calculations.