This step is crucial, as HFSP considers all unique site-pair combinations of the reactants or products to produce an exhaustive list of possible TS guesses. Note that the reactant and product geometries, a prerequisite for the HFSP algorithm, are sourced from Pynta's adsorbate optimization step, where Pynta attempts to optimize each unique adsorbate on all unique sites. To obtain a tractable set of transition states, we apply Pynta's harmonic filtering to remove overly strained structures that are likely to be either invalid or high-energy saddle points. For example, for the oxygen-assisted methanol oxidation step (reaction 1 in Table and the example shown in Figure ), HFSP begins with 129 TS guess geometries for the hydrogen atom transfer reaction from methanol to surface bound oxygen. After harmonic filtering and similarity analysis, 34 structures are retained. Figure shows the the number of HFSP configurations considered for all 53 reactions (∼1100 filtered HFSP configurations across the reaction set). The importance of considering many possible transition state guesses is demonstrated in the 0.25 eV range of transition state energies determined (Figure ).