Evaluating 0–0 Energies with Theoretical Tools: A Short Review

Abstract

For a given electronic excited state, the 0-0 energy (T 0 or T 00 ) is the simplest property allowing straightforward and physicallysound comparisons between theory and (accurate) experiment. However, the computation of 0-0 energies with ab initio approaches requires determining both the structure and the vibrational frequencies of the excited state, which limits the quality of the theoretical models that can be considered in practice. This explains why only a rather limited, yet constantly increasing, number of works have been devoted to the determination of this property. In this contribution, we review these efforts with a focus on benchmark studies carried out for both gas phase and solvated compounds. Over the years, not only as the size of the molecules increased, but the refinement of the theoretical tools has followed the same trend. Though the results obtained in these benchmarks significantly depend on both the details of the protocol and the nature of the excited states, one can now roughly estimate, in the case of valence transitions, the overall accuracy of theoretical schemes as follows: 1 eV for CIS, 0.2-0.3 eV for CIS(D), 0.2-0.4 eV for TD-DFT when one employs hybrid functionals, 0.1-0.2 eV for ADC (2) and CC2, and 0.04 eV for CC3, the latter approach being the only one delivering chemical accuracy on a near-systematic basis.[a] Dr.

Is ADC(3) as Accurate as CC3 for Valence and Rydberg Transition Energies?

“…Meaningful theory-experiment comparisons for ES are always challenging but the simplest and safest strategy is very likely to be comparing 0-0 energies, an approach that has been used many times before, e.g, see our recent review on the topic. 39 Following this strategy, we then consider here the (slightly extended) set of compounds defined in Ref. 35: it encompasses gas-phase measurements for 71 singlet and 30 triplet low-lying transitions.…”

A Mountaineering Strategy to Excited States: Highly Accurate Energies and Benchmarks for Medium Sized Molecules

“…37 They were, however, using experimental data as reference, which often precludes straightforward comparisons with theoretical vertical transition energies. 38,39 On the other hand, the second set, simply labeled as "radical", encompasses doubletdoublet transitions in radicals. We believe that the additional FCI-quality estimates that we provide in the present study for both types of compounds nicely complete our previous works and will be valuable for the electronic structure community.…”

The Quest for Highly Accurate Excitation Energies: A Computational Perspective

“…For a more faithful comparison between theory and experiment, although more computationally demanding, the so-called 0-0 energies are definitely a safer playground. [4][5][6][7] Second, developing theories suited for excited states is usually more complex and costly than their ground-state equivalent, as one might lack a proper variational principle for excited-state energies. As a consequence, for a given level of theory, excited-state methods are usually less accurate than their ground-state counterpart, potentially creating a groundstate bias that leads to inaccurate excitation energies.…”