Benchmarking for perturbative triple-excitations in EE-EOM-CC methods.

Area of Science:

• Computational chemistry
• Quantum chemistry
• Theoretical chemistry

Background:

• Equation of motion coupled cluster singles and doubles (EOM-CCSD) is a standard method for calculating excited states.
• Perturbative triples corrections ((T) and (T̃)) can enhance the accuracy of EOM-CCSD.
• Accurate calculation of vertical excitation energies is crucial for understanding molecular properties.

Purpose of the Study:

• To rederive and implement perturbative triples corrections ((T) and (T̃)) for EOM-CCSD.
• To assess the accuracy of these corrections by comparing them to the iterative EOM-CCSDT-3 method.
• To evaluate the performance and scalability of the implemented methods for larger molecular systems.

Main Methods:

• Rederi-vation and implementation of perturbative triples corrections ((T) and (T̃)) within the EOM-CCSD framework.
• Calculation of vertical excitation energies for a test set of molecules.
• Comparison of results with the iterative EOM-CCSDT-3 method.
• Analysis of computational performance and scaling on parallel systems.

Main Results:

• The EOM-CCSD(T) method demonstrated good agreement with EOM-CCSDT-3, with an average absolute deviation of 0.06 eV.
• The EOM-CCSD(T̃) method showed a larger deviation of 0.18 eV, suggesting issues with the handling of (T̃) terms.
• Scaling curves indicate the potential for routine application to larger molecular systems.

Conclusions:

• Perturbative triples corrections, particularly EOM-CCSD(T), offer an accurate and computationally feasible approach for studying molecular excited states.
• The EOM-CCSD(T̃) method requires further refinement due to observed inaccuracies.
• The developed parallel code enables efficient computation of excited states for increasingly complex molecules.