Theoretical analysis of the energetics and mechanism of a reaction can guide the selection of a catalyst from a set of similar candidates and avoid the need for lengthy experimental trials. In this work, a catalyst for the decarboxylation of acetic acid (AA) to methane and carbon dioxide was selected from a set of related magnesium hydroxide [Mg(OH)(2)](n) (n = 1-9) nanoclusters. Density functional theory (DFT) was used to follow the energetics, mechanism, and stereochemical details of the reaction. It was found that the n = 5 nanocluster had the best performance of the set. For this nanocluster, the decarboxylation reaction proceeded through a single transition state (TS), in contrast to an intermediate and two TSs for the free gas-phase catalytic reaction or decarboxylation with a (MgO)(4) catalyst. Inspection of AA adsorbed on the [Mg(OH)(2)](5) cluster shows the favorable structural orientation of the acid, which facilitated decarboxylation via a single activated state, bypassing the intermediate and one of the TSs. We hypothesized that the decarboxylation of propionic acid to ethane and carbon dioxide should also occur via a single TS with the same catalyst, which was confirmed by a separate DFT study. The [Mg(OH)(2)](5) clusters have potential use as a coating for textiles to catalyze the decomposition of propionic acid in sweat. [GRAPHICS] .