Calvin et al. [Proc. Natl. Acad. Sci. USA 121, e2307633121 (2024)] recently reported a result that sounds thermodynamically forbidden: semiconductor quantum dots with negative surface energy. If creating surface area lowers a system's free energy, why do these materials not spontaneously subdivide into oblivion? As Calvin et al. qualitatively recognized, this paradox vanishes because a ligand-capped nanocrystal is not a pristine solid-vacuum boundary, but a chemically open system where favorable ligand adsorption can drive the scalar surface excess negative. In this work, we formalize this physical distinction by extending the theory of continuum surface mechanics to account for chemically open, ligand-decorated surfaces. Our framework decouples the scalar calorimetric excess from its mechanical derivatives, illustrating that an apparent negative surface energy implies neither negative surface stress nor mechanical instability. Furthermore, this open-system formulation predicts fundamental chemomechanical phenomena: ligand relaxation softens tangent surface moduli, and adsorption-stress Maxwell relations explicitly link lattice strain to ligand coverage. Applying this theory, we demonstrate that nanostructure behavior is governed by the derivatives of the open-system interfacial free energy and finite-size corrections rather than the scalar energy alone. Open-system residual stress dictates lattice strain; favorable chemical interfacial energy drives core/shell wetting by overpowering positive elastic mismatch; and geometric regularizations, such as curvature, thermodynamically arrest runaway subdivision.

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• Continuum mechanics
• Quantum dots
• Materials modeling