Optimal-Reference Excited State Methods: Static Correlation at Polynomial Cost with Single-Reference Coupled-Cluster...

Area of Science:

• Quantum Chemistry
• Computational Chemistry
• Theoretical Chemistry

Background:

• Modeling chemical systems with static correlation in excited states is computationally demanding.
• Accurate methods often scale factorially, limiting their application.
• Existing methods may lack the precision for crucial photochemical processes.

Purpose of the Study:

• To explore the extension of single-reference coupled cluster theory to address static correlation in excited states.
• To develop an accurate and efficient method for modeling excited state properties.
• To investigate the utility of combining specific coupled cluster methods with the intermediate state representation (ISR).

Main Methods:

• Utilized "addition-by-subtraction" coupled cluster methods (pCCD, CCD0, CCD1, CCDf0/CCDf1) for ground state correlation.
• Combined these wave functions with the intermediate state representation (ISR).
• Developed and applied the CCDf1-ISR(2) approach for excited state calculations.

Main Results:

• The CCDf1-ISR(2) approach effectively handles static correlation.
• The method provides sufficient dynamical correlation for accurate excitation energy predictions (within ~0.2 eV for small organic molecules).
• Demonstrated advantages of the Hermitian ISR construction in avoiding failures seen in equation-of-motion methods.

Conclusions:

• The developed CCDf1-ISR(2) method offers a robust and accurate solution for excited state static correlation.
• Optimal single-reference theories leveraging initial wave function dependence show promise as economical approaches.
• Further exploration of these single-reference theories is warranted for efficient excited state modeling.