3.

(a) Distribution of the fine-tuning data set along the Ca–C distance used in each ECW-TL iteration. (b) Free-energy surfaces computed from the two DFT models and the embedded-DFT-SCAN fine-tuned models at successive iterations. Distinct ion-pair state regions (I), (II) and (III) are highlighted by different colors, with representative schematic structures shown on the right (Ca in green, C gray, O red, H white). The embedded-DFT-SCAN model systematically improves throughout the ECW-TL workflow and achieves near-reference accuracy by iteration 3 across all critical ion-pair states.