THE CORRELATED PI-HAMILTONIAN OF TRANS-BUTADIENE AS CALCULATED BY THE ABINITIO EFFECTIVE VALENCE SHELL HAMILTONIAN METHOD - COMPARISON WITH SEMIEMPIRICAL MODELS
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The pi-electron Hamiltonian of trans-butadiene is calculated using the effective Hamiltonian quasidegenerate many-body perturbation theory formalism to include "correlation" contributions. When four tight, valencelike orbitals are employed, the calculated reproduces experimental and calculated (by configuration interaction) valence state excitation energies, but fails to obtain the diffuse Rydberg-like pi-electron states which interleave the valence spectrum. The addition of a pair of Rydberg molecular orbitals to the valence shell leads to an effective Hamiltonian v which accurately describes both valence and Rydberg states simultaneously. These results provide an explanation as to why semiempirical pi-electron theories have required the use of different parameters, particularly resonance integrals, to calculate different properties. Our calculated contains hybrid, exchange, and multicenter two-electron integrals which are customarily ignored to varying degrees in zero differential overlap (ZDO) approximation based methods. The , integrals often differ considerably from their "theoretical," uncorrelated counterparts, and our , calculation provides further insight into these ZDO approximations. A more direct comparison is made with the traditional Pariser-Parr-Pople PPP by averaging our to generate a form similar to PPP. Certain aspects of are very close quantitatively to PPP, but there are other strong differences which are easily understood physically. The full , with all repulsion integrals and even effective three-electron interactions can now be used as a testing ground against which new types of semiempirical approximations may be tested. 1983 American Institute of Physics.
