Delocalization error poisons the density-functional many-body expansion

09 October 2024, Version 2
This content is a preprint and has not undergone peer review at the time of posting.

Abstract

The many-body expansion is a fragment-based approach to large-scale quantum chemistry that partitions a single monolithic calculation into manageable subsystems. This technique is increasingly being used as a basis for fitting classical force fields to electronic structure data, especially for water and aqueous ions, and for machine learning. Here, we show that the many-body expansion based on semilocal density functional theory affords wild oscillations and runaway error accumulation for ion–water interactions, typified by F–(H2O)N with N >= 15. We attribute these oscillations to self-interaction error in the density-functional approximation. The effect is minor or negligible in small water clusters, explaining why it has not been noticed previously, but grows to catastrophic proportion in clusters that are only moderately larger. This behavior can be counteracted with hybrid functionals but only if the fraction of exact exchange is >= 50%, whereas modern meta-generalized gradient approximations including wB97X-V, SCAN, and SCAN0 are insufficient to eliminate divergent behavior. Other mitigation strategies including counterpoise correction, density correction (i.e., exchange-correlation functionals evaluated atop Hartree-Fock densities), and dielectric continuum boundary conditions do little to curtail the problematic oscillations. In contrast, energy-based screening to cull unimportant subsystems can successfully forestall divergent behavior. These results suggest that extreme caution is required when the many-body expansion is combined with density functional theory.

Keywords

self-interaction error
delocalization error
many-body expansion
fragmentation
ion-water interactions
intermolecular interactions

Supplementary materials

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Additional data
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Molecular structures
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Cartesian coordinates for all structures
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