Rank-Reduced Equation-of-Motion Coupled Cluster Formalism with Full Inclusion of Triple Excitations

Area of Science:

• Computational Chemistry
• Quantum Chemistry
• Theoretical Chemistry

Background:

• Canonical equation-of-motion coupled cluster (EOM-CC) methods, while accurate, are computationally expensive, limiting their application to larger systems.
• Accurate calculation of molecular excited states is crucial for understanding photochemistry, spectroscopy, and material properties.

Purpose of the Study:

• To develop a rank-reduced variant of the equation-of-motion coupled cluster theory with singles, doubles, and triples excitations (EOM-CCSDT).
• To reduce the computational cost and storage requirements of EOM-CCSDT calculations while maintaining high accuracy for excited states.

Main Methods:

• Application of Tucker decomposition to the ground- and excited-states triply excited amplitude tensors.
• Exploitation of the linear scaling of decomposed amplitudes with system size (N) to achieve N^6 computational cost and N^4 storage.
• Systematic investigation of accuracy and performance across various molecular systems and excited state characters.

Main Results:

• The rank-reduced EOM-CCSDT method demonstrates significantly reduced computational cost and storage requirements compared to the canonical approach.
• The introduced error in the rank-reduced method is substantially smaller than the inherent error of the parent theory.
• Calculations for magnesium dimer and the NH3-F2 complex show accurate potential energy curves and spectroscopic parameters for various excited states.

Conclusions:

• The proposed rank-reduced EOM-CCSDT formalism provides a computationally feasible and accurate method for studying molecular excited states.
• The method's accuracy can be tuned by a single parameter, with the ability to recover the canonical method in a limiting case.
• This approach opens avenues for applying high-level coupled cluster theory to larger and more complex chemical systems.