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 (E<sup>vert</sup>), while
EOM-CCSD//TD-DFT calculations provide acceptable estimates for the S<sub>1</sub> relaxation energy (E<sup>relax</sup>), and (TD-)DFT suffices for the
S<sub>0</sub> → S<sub>1</sub> zero-point vibration energy correction
(∆E<sup>ZPVE</sup>). <div><br></div><div>Perturbative triples corrections deliver E<sup>vert</sup>
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 S<sub>0</sub> minima can be used to
calculate E<sup>vert</sup> with negligible loss in accuracy,
relegating the O(N<sup>5</sup>) expense of CCSD to only
single-point energy and excitation calculations. </div><div><br></div><div>This affordable protocol can be applied to all
volatile carbonyl species.
E<sup>0−0</sup> 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.</div>
