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Application of Correlated-Wavefunction and Density-Functional Theories to Endofullerenes: A Cautionary Tale

K. Panchagnula, D. Graf, K. R. Bryenton, D. P. Tew, E. R. Johnson, A. J. W. Thom

TL;DR

This work targets the challenge of accurately describing the symmetric double-well potential along the anisotropic axis of the endofullerene Ne@C70, where diverse electronic-structure methods often disagree. By computing new correlated-wavefunction data via DLPNO-CCSD(T0) and benchmarking a broad set of DFAs with various dispersion corrections against these references, the authors quantify how $BH$ (barrier height) and $z_m$ (minima position) depend on method, grid, and damping. They show that many functionals are highly sensitive to numerical integration grids and dispersion-damping choices, with some yielding spurious oscillations and others aligning more closely with WF references (notably MP2 over RPA@PBE). The DLPNO-WF results suggest the true $BH$ lies in the tens of $cm^{-1}$, closer to MP2 than to RPA@PBE, underscoring that dispersion-corrected DFT remains a delicate tool for this system. The study concludes that Ne@C70 is an excellent diagnostic for testing correlation methods and dispersion corrections, and it calls for more experimental data to constrain and guide methodological development.

Abstract

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.

Application of Correlated-Wavefunction and Density-Functional Theories to Endofullerenes: A Cautionary Tale

TL;DR

This work targets the challenge of accurately describing the symmetric double-well potential along the anisotropic axis of the endofullerene Ne@C70, where diverse electronic-structure methods often disagree. By computing new correlated-wavefunction data via DLPNO-CCSD(T0) and benchmarking a broad set of DFAs with various dispersion corrections against these references, the authors quantify how (barrier height) and (minima position) depend on method, grid, and damping. They show that many functionals are highly sensitive to numerical integration grids and dispersion-damping choices, with some yielding spurious oscillations and others aligning more closely with WF references (notably MP2 over RPA@PBE). The DLPNO-WF results suggest the true lies in the tens of , closer to MP2 than to RPA@PBE, underscoring that dispersion-corrected DFT remains a delicate tool for this system. The study concludes that Ne@C70 is an excellent diagnostic for testing correlation methods and dispersion corrections, and it calls for more experimental data to constrain and guide methodological development.

Abstract

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. 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 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.

Paper Structure

This paper contains 4 sections, 2 equations, 2 figures, 2 tables.

Table of Contents

  1. Introduction
  2. Theory
  3. Results
  4. Conclusion

Figures (2)

  • Figure 1: PES slices of (a) WF methods and (b) DFAs for points separated by 0.02Å. For the WF methods, MP2 is shown in blue and RPA@PBE in maroon; DLPNO single points are shown as crosses with error bars with MP2, CCSD and CCSD(T0) in green, purple and pink respectively. For the DFAs, GGAs and GGA-based hybrids are given by solid lines, mGGA-based hybrids by dashed lines, and double hybrids by dotted lines. HF, PBE, B97, $\omega$-B97X, B97M-V, $\omega$-B97M-V, $\omega$-B97X-V, B2PLYP-D3(BJ), PWPB95-D3(BJ), $\omega$-B97M(2) are in blue, maroon, light green, purple, forest green, pink, yellow, light blue, brown, and turquoise.
  • Figure 2: PES slices of (a) PBE0 with dispersion corrections D3(0), D3(BJ), XDM(BJ), MBD@rsSCS, MBD-NL, XDM(Z), and TS in forest green, pink, turquoise, purple, green, yellow and maroon respectively; (b) XDM(BJ) as solid and XDM(Z) as dashed lines added on top of PBE0, B86bPBE0, B86bPBE-50 and LC-$\omega$PBEh base functionals in maroon, green, purple and forest green respectively. MP2 is shown on both panels in blue as a reference WF method for comparison.