In addition to activation energy, activation volume also provides information about the transition state, arguably, in a more tangible manner. The volume/size of a TS structure is directly related to its geometry. While competing mechanisms of a reaction can sometimes be difficult to distinguish based on activation energies, they may be distinguishable by activation volumes when the competing TS structures have very different geometries. This is especially useful in studying the stepwise vs concerted mechanisms of cycloadditions, given that the stepwise mechanism often has a less negative activation volume. In order to calculate the activation volume of a reaction, the barriers computed by XP-PCM are plotted against the pressure (Figure ). The ∆𝐺 232 ‡ (𝑝) data shows a nice linear relationship. The slope of the fitted line gives the activation volume, according to eq. ( ) .