, where ∆𝐺 ‡ is the Gibbs energy of activation, according to the thermodynamic formulation of the transition state theory. While activation energies are almost always positive, activation volumes can commonly be both positive and negative. Reactions with negative and positive ∆𝑉 ‡ are accelerated and decelerated by pressure, respectively. Usually, the signs of activation volumes can be intuitively predicted. For example, in the case of a bond formation reaction between two molecules (Figure ), a negative ∆𝑉 ‡ is expected since the unimolecular transition state is more compact and hence has a smaller volume than the two separated reactant molecules. The opposite is usually true for bond cleavage reactions.