Analysis of the effect of changing the a0 parameter of the Becke3-LYP hybrid functional on the transition state geometries and energy barriers in a series of prototypical reactions

Abstract

A series of eleven gas-phase chemical reactions have been examined to assess the dependence of transition state geometries and energy barriers, as well as energy differences between reactants and products, on the a₀ B3LYP functional parameter. Throughout the study we have changed the a₀ parameter from 0.1 to 0.9 and for the a_c and a_x parameters we have followed the relationships a_x = 1 − a₀ and a_c = a_x. By comparing with the QCISD transition state geometries and energy barriers, our systematic study allows us to identify the influence of the a₀ parameter in the reactions studied. In general, B3LYP calculations with the original parameters underestimate energy barriers, this trend being corrected when the a₀ parameter increases. Our study also shows that the fraction of Hartree–Fock exchange needed to predict accurate barrier heights differs from the optimal fraction needed to predict thermochemical properties and geometries.