State-Specific Coupled-Cluster Methods for Excited States

Area of Science:

• Quantum Chemistry
• Computational Chemistry
• Theoretical Chemistry

Background:

• The standard Equation-of-Motion Coupled-Cluster with Single and Double excitations (EOM-CCSD) method struggles with doubly excited states.
• The ΔCCSD method, utilizing non-Aufbau determinants, offers an alternative for targeting excited states, particularly doubly excited ones.

Purpose of the Study:

• To benchmark the accuracy and consistency of the ΔCCSD method against EOM-CCSD for various types of molecular excited states.
• To assess the performance of ΔCCSD beyond doubly excited states, including doublet-doublet transitions and singly excited states of closed-shell systems.

Main Methods:

• Comparison of ΔCCSD and EOM-CCSD methods for computing excitation energies.
• Utilized a dataset of 276 excited states from the quest database.
• Employed a minimalist two-determinant coupled-cluster approach for singly excited states in closed-shell systems.

Main Results:

• ΔCCSD shows effectiveness for doubly excited states but generally underperforms EOM-CCSD for other excitation types.
• Mean absolute errors (MAEs) for doublet-doublet transitions were 0.10 eV (ΔCCSD) vs. 0.07 eV (EOM-CCSD).
• MAEs for singly excited states were 0.15 eV (ΔCCSD) vs. 0.08 eV (EOM-CCSD), with higher multiconfigurational character contributing to ΔCCSD's lower accuracy.

Conclusions:

• ΔCCSD is a viable method for doubly excited states where EOM-CCSD falters.
• For most other excited states, EOM-CCSD remains the more accurate and consistent choice.
• State-specific optimized orbitals provide marginal improvements for ΔCCSD accuracy.