DFT Comparison of Exact and Approximate Free Energy Calculations for Polyatomic Adsorbates

The objective of this work is to compare the various adsorbate free energy approximations by comparing them against a full PES sampling method. Furthermore, we show the impact of free energy model choice on equilibrium predictions of a catalytic reaction model involving polyatomic adsorbates. Focus is placed on comparing the harmonic oscillator and free translator (HO,FT) free energy approximations. The procedure for performing DFT calculations and computing free energies will be given initially. Next, the formulation of the equations of an equilibrium model will be derived and we will explain possible limitations of the model due to the assumptions used. Finally, we will quantify the differences in adsorbate free energy approximations by means of direct comparison and by looking at the sensitivity of model results to free energy calculation method.