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

Composite recipes based on ground-state coupled cluster methods can be used to calculate thermochemically relevant quantities such as heats of formation and ionization potentials to within 1 kJ/mol of experimental values. Achieving this level of accuracy with composite recipes typically requires inclusion of electron correlation of up to quadruple excitations. In this study, composite recipes based on the equation of motion coupled cluster theory (EOM-CC) have been designed to calculate electronic excitation energies of cations and ionization potentials (IP) of closed-shell molecules. The adiabatic ionization energies were calculated using two different recipes, and the results were compared to experimental values. With the more rigorous recipe, predictions of excitation and ionization energies were consistently within subchemical accuracy, with the ionization energies predicted with a mean absolute error (MAE) of 25 cm^-1. An alternative recipe, designed to be more affordable, was applied to calculate IPs for a set of 16 molecules; it predicted ionization energies with an MAE of 132 cm^-1. The level of electron correlation required in these recipes to reach subchemical accuracy is indicative of the slower convergence of EOM-CC to the full configuration interaction limit compared to ground-state CC methods.