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Charging free energies for sodium and chloride. Affine-response model captures a portion of the FEP results but not over the entire range of q; piecewise response is needed to fit over the whole range. Inset: The single-coefficient response model (small symbols) reproduces the FEP results less accurately; the model parameters were determined separately over the intervals q > 0 and q < 0 but constrained to have the same static potential.
