An Energy Decomposition Method for Electronic Excitations in Two-Determinant Restricted Open-Shell Kohn-Sham Theory

Area of Science:

• Computational Chemistry
• Quantum Chemistry
• Theoretical Chemistry

Background:

• Restricted open-shell Kohn-Sham (ROKS) theory is crucial for describing single-electron excitations.
• Conventional single-determinant energy decomposition analysis (EDA) methods struggle with the two-determinant nature of ROKS excited states.
• Accurate analysis of ROKS excited states is essential for understanding electronic excitations.

Purpose of the Study:

• To develop a novel EDA framework tailored for ROKS excited states.
• To enable detailed analysis of electronic excitations by dissecting them into primary orbital excitation and secondary relaxation components.
• To provide chemical insights into relaxation processes and charge redistribution during electronic excitations.

Main Methods:

• Developed a new EDA framework for ROKS states by transforming the problem into an effective single-determinant form.
• Applied Natural Orbitals for Chemical Valence (NOCV) and Occupied-Virtual Orbitals for Chemical Valence (OVOCV) theories.
• Separated electronic excitations into a primary excitation and secondary polarization/relaxation, quantifying energy changes and electron promotion numbers.

Main Results:

• The new ROKS excitation EDA framework successfully analyzes electronic excitations.
• The method decomposes excitations into primary orbital changes and secondary relaxation effects, quantifying their energetic contributions.
• The OVOCV approach further separates relaxation into ranked occupied-to-virtual contributions, revealing detailed charge redistribution.

Conclusions:

• The developed ROKS excitation EDA provides unprecedented chemical insights into relaxation processes and charge redistribution.
• The framework is demonstrated on various systems, including valence and core excitations, charge-transfer, and intramolecular charge transfer (ICT) states.
• This method offers a powerful tool for understanding the complex nature of electronic excitations in diverse chemical systems.