Activation volume is a crucial concept in mechanochemistry because it describes how the applied mechanical energy facilitates chemical reactions. The activation volume can be determined readily from the load dependence of a chemical reaction based on the stress-assisted thermal activation theory (also known as the Bell model), but it is still very challenging to fully understand what activation volume means physically and how to compute it. Here, by introducing a concept of inherent stress difference (${\sigma }_{\mathrm{T}}$) between the initial and transition states, which is associated with the displacement of reacting atoms, a universal formula of activation volume ($\mathrm{\Delta }{V}^{*}$) for the interfacial reactions is derived, that is, $\mathrm{\Delta }{V}^{*}=(V_{\mathrm{eff}}{\sigma }_{\mathrm{T}}/E)$, where $E$ and $V_{\mathrm{eff}}$ refer to the Young’s modulus and the volume influenced by reaction, respectively. This formula implicates that $\mathrm{\Delta }{V}^{*}$ is related to the volume change from the initial to transition states. Then, the first-principles calculation was used to confirm the validity of the proposed definition in diverse types of chemical reactions. This Letter provides clear physical insight into how a chemical reaction that is unlikely to occur under thermal conditions is facilitated by mechanical stress, explaining the core principle of mechanochemistry.

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• Elasticity
• First-principles calculations
• Material failure
• Stress