CHEMICAL HARDNESSES OF ATOMS AND MOLECULES FROM FRONTIER ORBITALS

It is demonstrated that the chemical hardness of molecules is exactly the screened interaction energy of the electrons in the frontier (HOMO , LUMO) orbitals, whereas the derivative of the Kohn-Sham potential re lative to the electron number is a screened local hardness measuring t he local reactivity of molecules and solids. It is also shown that the hardness of two interacting molecules involves the screened interacti on between their respective HOMO and LUMO orbitals. The formula are ap plied to the Thomas-Fermi model. (C) 1997 Published by Elsevier Scienc e B.V.