Application of Correlated-Wavefunction and Density-Functional Theories to Endofullerenes: A Cautionary Tale
A recent study by Panchagnula et al. [J. Chem. Phys. 161, 054308 (2024)] illustrated the non-concordance of a variety of electronic structure methods at describing the symmetric double-well potential expected along the anisotropic direction of the endofullerene Ne@C$_{70}$. In this article we present new correlated-wavefunction data from coupled cluster theory for this system, and scrutinise a variety of state-of-the-art density-functional approximations (DFAs) and dispersion corrections (DCs). We identify rigorous criteria for the double-well potential and compare the shapes, barrier heights, and minima positions obtained with the DFAs and DCs to the correlated wavefunction data. We show that many of the DFAs are extremely sensitive to the numerical integration grid used, the dispersion damping function, and the extent of exact-exchange mixing. We pose the Ne@C$_{70}$ system as a challenge to functional developers and as a diagnostic system for testing dispersion corrections, and reiterate the need for more experimental data for comparison.