DENSITY-FUNCTIONAL APPROACH TO HARDNESS EVALUATION AND ITS USE IN THESTUDY OF THE MAXIMUM HARDNESS PRINCIPLE

An alternative method for computing chemical hardness, based on the Ja nak's extension of density-functional theory for fractional occupancie s? 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 hardness es 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 che mical potential. The electronic and nuclear-repulsion energy changes s how good correlation with the relative stability of a species even whe n the constraint of constant chemical potential is not obeyed.