Implementation of an Ellipsoidal-Cavity Field Correction for Computed Molecular Oscillator Strengths in Solution: A(nother) Benchmark Study

We recently compared
oscillator strengths (OS) obtained
from electronic
structure calculations (fcomp) to OSs
derived from experimental spectra (fexp) multiplied by the refractive index (n) of the
solution in which the spectra were measured. The choice of nfexp instead of fexp as a reference accounts for the macroscopic flux of energy in a
dielectric (the experimental solvent). Here, we apply an approximate
correction to fcomp values that accounts
for the local electromagnetic field driving the absorption transition
(which is generally different from the macroscopic field). We refer
to these modified OSs as fcompS. The correction is obtained by assuming
that each molecule occupies an ellipsoidal cavity, fitted to its van
der Waals surface, surrounded by a continuum dielectric model representing
the solvent. Sets ranging from 33 to 85 experimental transitions are
used for the benchmark. For LR-CCSD and EOM-CCSD, we find that fcompS generally gives a better agreement with experimental strengths than fcomp. For LR-CCSD in the length gauge, for instance,
there is a 1 to 1 scaling of the (nfexp, fcompS) pairs. Instead, the results for TD-DFT depend on the amount
of HF exchange used in the functional: pure functionals typically
also have a 1 to 1 scaling of the (nfexp, fcompS) pairs, while for hybrid functionals fcompS overestimates nfexp to a degree that appears proportional to
the amount of HF exchange present in the functional.