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

Abstract

High Resolution Image Download MS PowerPoint Slide 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.

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