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

Abstract

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.