Electronic Excitation Energies and Ionization Potentials with Sub-Chemical Accuracy from EOM-CC Composite Methods

Area of Science:

• Computational Chemistry
• Quantum Chemistry
• Theoretical Spectroscopy

Background:

• Composite recipes using ground-state coupled cluster (CC) methods achieve high accuracy (within 1 kJ/mol) for thermochemical properties.
• These methods typically require electron correlation up to quadruple excitations for high accuracy.
• Equation of Motion Coupled Cluster (EOM-CC) theory is a powerful tool for studying excited states and ionization processes.

Purpose of the Study:

• To design and evaluate composite recipes based on EOM-CC theory for calculating electronic excitation energies of cations.
• To determine ionization potentials (IP) of closed-shell molecules using EOM-CC composite recipes.
• To assess the accuracy and efficiency of different EOM-CC recipes for these calculations.

Main Methods:

• Development of two composite recipes utilizing EOM-CC theory.
• Calculation of adiabatic ionization energies for a set of closed-shell molecules.
• Comparison of calculated ionization energies with experimental data.

Main Results:

• A rigorous EOM-CC recipe achieved subchemical accuracy for excitation and ionization energies, with a Mean Absolute Error (MAE) of 25 cm^-1 for ionization energies.
• A more computationally affordable EOM-CC recipe predicted ionization energies with an MAE of 132 cm^-1 for 16 molecules.
• The results indicate that EOM-CC methods require significant electron correlation for subchemical accuracy, showing slower convergence to the full configuration interaction limit compared to ground-state CC methods.

Conclusions:

• EOM-CC composite recipes are effective for accurately predicting electronic excitation energies and ionization potentials.
• The choice of recipe impacts the trade-off between accuracy and computational cost.
• The convergence behavior of EOM-CC highlights the importance of electron correlation in achieving high accuracy.