Predicting Carbonyl Excitation Energies Efficiently Using EOM-CC Trends

We approach the problem of predicting excitation energies of diverse, larger (5–6 carbons) carbonyl species central to earth’s tropospheric chemistry. Triples contributions are needed for the vertical excitation energy (Evert), while EOM-CCSD//TD-DFT calculations provide acceptable estimates for the S1 relaxation energy (Erelax), and (TD-)DFT suffices for the S0 → S1 zero-point vibration energy correction (∆EZPVE). Perturbative triples corrections deliver Evert values close in accuracy to full iterative triples EOM-CC calculations. The error between EOM-CCSD and triples-corrected E vert values appears to be systematic and can be accounted for with scaling factors. However, saturated and α,β-unsaturated carbonyls must be treated separately. Double-hybrid S0 minima can be used to calculate Evert with negligible loss in accuracy, relegating the O(N5) expense of CCSD to only single-point energy and excitation calculations. This affordable protocol can be applied to all volatile carbonyl species. E0−0 predictions do overestimate measured values by ∼8 kJ/mol due to a lack of triples contribution in E relax, but this overestimation is systematic and the mean unsigned error is within 4 kJ/mol once this is accounted for.