Electron Alchemy with Machine-Learned Interatomic Potentials: Case Studies of Local Charge in Bond Dissociation Curves.
The convergence of molecular dynamics simulations and machine-learned interatomic potentials (MLIPs) promises density functional theory (DFT) level accuracy at near-classical force-field computational costs. However, while the average fidelity to reference energies and forces approaches perfection, several failure modes limit MLIP reliability in production simulations. These include spurious bond formation, inconsistent reproduction of long-range interactions, and inconsistent spin-state references. Here, the origins of these behaviors are studied by benchmarking the UMA, ORB, MACE, and AIMNet2 models against reference DFT bond dissociation curves for an illustrative range of species. These benchmarks reveal that models without explicit atomic charge resolution predict spurious stable bonds between like-charged halide anions, effectively transmuting two Cl- ions into neutral Cl2. Models with atomic partial charge equilibration correctly predict repulsion in these systems. Conversely, several secondary limitations are exposed in these benchmarks, including inconsistent agreement with unrestricted DFT (uDFT) versus restricted DFT (rDFT) energies and inconsistent core-region treatment. This comparative analysis suggests that, while artifacts related to core repulsion and asymptotic electrostatics are readily repairable through improved physical priors and better data curation, the issue of spurious bond formation is intrinsic to the inability of global charge specification to disambiguate similar local geometries at different charge and spin states.