Effects of cluster size on calculation of activation energies of silicon surface reactions with H₂ and HCl

In order to determine the effect of cluster size on the calculation of activation energies of reactions at silicon surfaces, intradimer adsorption/desorption reactions of H₂ and HCl with silicon clusters of various sizes and shapes were analyzed under the B3LYP/6-31G(d,p) level. Activation energies of reactions of H₂ and HCl with clusters composed of different number and orientation of dimers on the surface were estimated. For clusters having dimers parallel to each other, the activation energies converged satisfactorily at three dimers, but for clusters having dimers aligned in a row, activation energies did not converge sufficiently even at four dimers. The activation energies for clusters having three dimers parallel to each other on the surface were seen to be in good agreement with values published in the literature, but considering potential application of the cluster to model intrarow and interrow reaction mechanisms, reactions of H₂ and HCl with a cluster having three dimers parallel to each other and three dimers aligned in a row on the surface were also investigated, and the resultant activation energies were in even better agreement with published values.