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 (<i>f</i>_comp) to OSs derived from experimental spectra (<i>f</i>_exp) multiplied by the refractive index (<i>n</i>) of the solution in which the spectra were measured. The choice of <i>nf</i>_exp instead of <i>f</i>_exp as a reference accounts for the macroscopic flux of energy in a dielectric (the experimental solvent). Here, we apply an approximate correction to <i>f</i>_comp 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 <i>f</i>_comp^S. 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 <i>f</i>_comp^S generally gives a better agreement with experimental strengths than <i>f</i>_comp. For LR-CCSD in the length gauge, for instance, there is a 1 to 1 scaling of the (<i>nf</i>_exp, <i>f</i>_comp^S) 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 (<i>nf</i>_exp, <i>f</i>_comp^S) pairs, while for hybrid functionals <i>f</i>_comp^S overestimates <i>nf</i>_exp to a degree that appears proportional to the amount of HF exchange present in the functional.