Excited states with internally contracted multireference coupled-cluster linear response theory

Area of Science:

• Quantum Chemistry
• Computational Chemistry
• Theoretical Chemistry

Background:

• Accurate computation of excitation energies is crucial for understanding molecular properties.
• Systems with pronounced multireference character pose significant challenges for standard quantum chemical methods.
• Existing methods may suffer from issues like spurious roots or limitations in applicability.

Purpose of the Study:

• To formulate and implement a linear response (LR) theory for internally contracted multireference coupled cluster (ic-MRCC).
• To accurately calculate excitation energies for ground states with significant multireference character.
• To address limitations of previous methods and provide a robust approach for electronic excited states.

Main Methods:

• Formulation of linear response (LR) theory for internally contracted multireference coupled cluster (ic-MRCC).
• Implementation of the ic-MRCC singles-and-doubles (ic-MRCC-SD) framework.
• Adoption of a Tamm-Dancoff-type approximation to handle unphysical poles, consistent with equation-of-motion derivations.

Main Results:

• The developed ic-MRCC-LR method accurately computes excitation energies for various molecules, including CH2, p-benzyne, and conjugated polyenes.
• The method performs well even for difficult cases like doubly excited states.
• The approach is applicable to open-shell ground states and avoids biases towards specific reference determinants, unlike some other methods.

Conclusions:

• The implemented ic-MRCC-LR theory provides a reliable and accurate tool for calculating excitation energies in challenging multireference systems.
• The method overcomes issues such as spurious roots encountered in related theories (e.g., Mk-MRCC-LR).
• This advancement offers a more robust computational approach for studying the electronic excited states of molecules.