Studying excited-state-specific perturbation theory on the Thiel set

Area of Science:

• Computational Chemistry
• Quantum Chemistry
• Theoretical Chemistry

Background:

• Excited-state calculations are crucial for understanding molecular properties and reactions.
• Accurate and efficient methods are needed to study electronic excitations.

Purpose of the Study:

• To evaluate the performance of the N5-scaling excited-state-specific second order perturbation theory (ESMP2) for singlet excitations.
• To compare ESMP2 with other established computational methods on the Thiel benchmarking set.

Main Methods:

• Application of ESMP2 with and without regularization to the Thiel set of molecules.
• Comparison of ESMP2 results against CC2, EOM-CCSD, CC3, and TD-DFT methods.
• Analysis of the ESMP2 doubles norm for detecting doubly excited character.

Main Results:

• Unregularized ESMP2 shows high sensitivity to π system size.
• Regularized ESMP2 demonstrates reduced sensitivity to π system size and improved accuracy.
• Regularized ESMP2 outperforms CC2, EOM-CCSD, CC3, and TD-DFT on the Thiel set.
• ESMP2 doubles norm provides a low-cost indicator of doubly excited states.

Conclusions:

• Regularized ESMP2 offers a robust and accurate approach for studying singlet excitations.
• ESMP2 presents a competitive alternative to existing methods, particularly for systems where π conjugation varies.
• The ESMP2 doubles norm is a valuable tool for characterizing excited states without requiring active space definitions.