ITERATIVE EXTENDED HÜCKEL THEORY
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Date
1966
Journal Title
Journal ISSN
Volume Title
Publisher
Ohio State University
Abstract
The extended H\""{u}ckel MO theory is modified to improve its description of the electronic structures of molecules including heteroatoms, by making the orbital ionization potentials $(\alpha)$ linearly dependent upon the net atomic charges (q). That is, $\alpha = \alpha_{0} + q \Delta \alpha$, where $\alpha_{0}$ and $\Delta \alpha$ depend upon the kind of atom involved. Since the atomic charges are found by solution of the MO problem, an iterative technique is needed to find mutually consistent q and $\alpha$ values. A damped iteration scheme was found to converge well, normally requiring 5-15 iterations to give charges to $\pm 0.01$ unit. We used Hoffmann's computer $program^{1}$ as revised by Cusachs and further modified by us to include the iterative feature. The resonance integrals ($\beta$) were obtained for each iteration from the $\alpha$ values then in use, using Cusachs' $formula.^{2}$. A systematic study was carried out to find a single optimum set of $\alpha_{0}$ and $\Delta \alpha$ values for each state of each kind of atom, using dipole moments and molecular ionization potentials as criteria for parameter selection. The parameter sets found for C. N. H. and O are satisfactory for a group of compounds exhibiting a wide range of bonding and geometrical features. This work was supported in part by the National Institute of Health, the National Science Foundation and the National Aeronantics and Space Administration.
Description
$^{1}$ R. Hoffmann, J. Chem. Phys. 39, 1307 (1963). $^{2}$ L. C. Cusachs, J. Chem. Phys. 43, 8157 (1965).
Author Institution: Theoretical Biology Center; Department of Chemistry, State University of New York at Buffalo; Department of Chemistry, Stanford University
Author Institution: Theoretical Biology Center; Department of Chemistry, State University of New York at Buffalo; Department of Chemistry, Stanford University