Insights for an Accurate Comparison of Computational
Data to Experimental Absorption and Emission Spectra: Beyond the Vertical
Transition Approximation
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Abstract
In this work we carefully investigate
the relationship between
computed data and experimental electronic spectra. To that end, we
compare both vertical transition energies, <i>E</i><sub>V</sub>, and characteristic frequencies of the spectrum like the
maximum, ν<sup>max</sup>, and the center of gravity, <i>M</i><sup>1</sup>, taking advantage of an analytical expression
of <i>M</i><sup>1</sup> in terms of the parameters of the
initial- and final-state potential energy surfaces. After pointing
out that, for an accurate comparison, experimental spectra should
be preliminarily mapped from wavelength to frequency domain and transformed
to normalized lineshapes, we simulate the absorption and emission
spectra of several prototypical chromophores, obtaining lineshapes
in very good agreement with experimental data. Our results indicate
that the customary comparison of experimental ν<sup>max</sup> and computational <i>E</i><sub>V</sub>, without taking
into account vibrational effects, is not an adequate measure of the
performance of an electronic method. In fact, it introduces systematic
errors that, in the investigated systems, are on the order of 0.1–0.3
eV, i.e., values comparable to the expected accuracy of the most accurate
computational methods. On the contrary, a comparison of experimental
and computed <i>M</i><sup>1</sup> and/or 0–0 transition
frequencies provides more robust results. Some rules of thumbs are
proposed to help rationalize which kind of correction one should expect
when comparing <i>E</i><sub>V</sub>, <i>M</i><sup>1</sup>, and ν<sup>max</sup>