Density-Functional Approach to Hardness Evaluation and Its Use in the Study of the Maximum Hardness Principle

An alternative method for computing chemical hardness, based on the Janak's extension of density-functional theory for fractional occupancies, is employed in the study of the maximum hardness principle for HCN, HSiN, N2H2, HCP, and O3H+ isomerizations. The hardness is found to be a good indicator of the more stable isomer in all cases. The hardnesses and the energy profiles, as a function of the reaction coordinate, are generally opposite in nature only for the isomerization reactions of O3H+ and HSiN, for which there is a negligible variation of the chemical potential. The electronic and nuclear-repulsion energy changes show good correlation with the relative stability of a species even when the constraint of constant chemical potential is not obeyed.