On the use of the iterative extended Hückel method and model Hamiltonians in chemisorption theory

It is shown that current versions of the extended Hückel method that include self-consistency of charge have a principle defect, because screening of the intra-atomic electron repulsion due to interatom electron–electron interactions is neglected. A similar shortcoming exists for Anderson and Hubbard model calculations of covalent chemical bonding. Using the Roothaan equations it is demonstrated that the screening due to interatom electron-electron interactions depends on the bond order between the atoms. Hence transferability of effective intra-atomic repulsion integrals in calculations where chemical bonding changes considerably is not justified. CNDO and INDO semiempirical methods are shown to be consistent with first order in overlap approximations to the interatomic electron repulsion integrals.