Excited States by Coupling Piris Natural Orbital Functionals with the Extended Random-Phase Approximation

Area of Science:

• Quantum Chemistry
• Computational Chemistry
• Theoretical Chemistry

Background:

• Calculating excited-state energies is crucial for understanding molecular properties and reactions.
• Traditional methods can be computationally expensive for larger systems.
• Piris natural orbital functionals (PNOFs) offer a promising avenue for electronic structure calculations.

Purpose of the Study:

• To develop and evaluate a novel method, PNOF-ERPA, for computing excited-state energies.
• To assess the performance of different PNOF variants (PNOF5, PNOF7, GNOF) in conjunction with ERPA.
• To investigate the impact of electron correlation on excited-state energy calculations.

Main Methods:

• Coupling reconstructed second-order reduced density matrices from PNOFs with the extended random-phase approximation (ERPA).
• Implementing and testing PNOF-ERPA, including specific variants PNOF-ERPA0, PNOF-ERPA1, and PNOF-ERPA2.
• Comparing results against the established configuration interaction (CI) method for validation.

Main Results:

• The PNOF-ERPA method demonstrated good accuracy for the first excited states of small molecules (H2, HeH+, LiH, Li2, N2).
• Accuracy improved with higher-order ERPA approximations (ERPA0 < ERPA1 < ERPA2).
• Global NOF (GNOF) provided better results for larger systems by including more electron correlation, while PNOF5 was effective for smaller systems.

Conclusions:

• The extension of PNOFs to excited-state calculations via PNOF-ERPA is successful.
• PNOF-ERPA presents a viable and promising computational method for future applications in excited-state chemistry.
• The choice of PNOF functional is important for accuracy, particularly concerning electron correlation in larger molecules.