In the Hükkel and other methods, binding energies are calculated by subtracting the sum of orbital electronic energies for the molecule from the sum of orbital electronic energies for the separated atoms, and not considering the internuclear repulsion. Since this last may be several orders of magnitude greater than the binding energy, reasonable results could not be obtained without an approximate cancellation with another neglected term. It is shown that such a cancellation is a consequence of the isoelectronic principle (invariance of binding energy to change in atomic number of constituent atom). Numerical examples are given.
Goodisman, Jerry, "The Isoelectronic Principle and the Accuracy of Binding Energies in the Hückel Method" (1969). Chemistry Faculty Scholarship. 45.
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