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