Excitation Energies from the Entanglement Coupled Cluster Model for Doublets

Area of Science:

• Quantum Chemistry
• Computational Chemistry
• Molecular Spectroscopy

Background:

• Calculating excitation energies for open-shell molecular systems, particularly doublets, presents significant challenges in quantum chemistry.
• Traditional spin-adaptation methods often struggle with complexity and convergence for open-shell electronic structures.

Purpose of the Study:

• To develop a novel spin-adapted coupled cluster singles and doubles (CC2) model for accurate calculation of excitation energies in molecular doublets.
• To address the limitations of existing methods in handling spin adaptation for open-shell species.

Main Methods:

• Introduction of the entanglement coupled cluster (ECC) approach, which couples open-shell systems to bath orbitals to form a closed-shell system.
• Utilizing a spin-adapted CC2 framework within the ECC methodology.
• Retaining key features of closed-shell CC methods, such as a clear cluster operator definition and a terminating Baker-Campbell-Hausdorff expansion.

Main Results:

• The ECC-CC2 model yields excitation energies of quality comparable to established closed-shell coupled cluster methods.
• The developed model demonstrates improved accuracy for certain ionized states compared to the equation-of-motion approach for ionized states.

Conclusions:

• The entanglement coupled cluster singles and doubles model provides a robust and accurate method for computing excitation energies of molecular doublets.
• This new approach offers a viable alternative for studying open-shell systems, particularly for challenging ionized states.