Recovering dynamic correlation in spin flip configuration interaction through a difference dedicated approach.

Area of Science:

• Quantum chemistry
• Computational chemistry
• Theoretical chemistry

Background:

• Accurate treatment of polyradical systems and multi-excitonic states is computationally challenging.
• Existing spin flip methods have limitations in capturing dynamic correlation.
• Understanding singlet fission mechanisms requires precise energetics of excited states.

Purpose of the Study:

• Introduce and validate new restricted-active-space n-spin flip configuration interaction models (RAS(S)-SF and RAS(S,2h,2p)-SF).
• Assess the performance of these models in describing ground and excited states of various chemical systems.
• Investigate the applicability of these methods to large systems relevant to singlet fission.

Main Methods:

• Development of restricted-active-space n-spin flip configuration interaction models.
• Application of RAS(S)-SF and RAS(S,2h,2p)-SF to methylene, tetramethyleneethane, transition metal complexes, and tetracene dimers.
• Comparison with the existing RAS(h,p)-SF method to evaluate dynamic correlation effects.

Main Results:

• RAS(S,2h,2p)-SF significantly improves state descriptions across all tested systems.
• High accuracy is achieved even with a minimal number of spin flips.
• The double triplet state (1(TT)) in tetracene dimers is predicted to be unbound using a triple-zeta basis, contradicting previous studies.

Conclusions:

• The new RAS(S)-SF and RAS(S,2h,2p)-SF models provide accurate and computationally efficient approaches for polyradical systems.
• RAS(S,2h,2p)-SF offers substantial improvements in describing electronic states.
• The findings provide crucial insights into the energetics of tetracene dimers and their potential for efficient singlet fission.