The correlated pihyphen;Hamiltonian oftranshyphen;butadiene as calculated by theab initioeffective valence shell Hamiltonian method: Comparison with semiempirical models

摘要

The pihyphen;electron HamiltonianHpgr;oftranshyphen;butadiene is calculated using the effective Hamiltonian quasidegenerate manyhyphen;body perturbation theory formalism to include lsquo;lsquo;correlationrsquo;rsquo; contributions. When four tight, valencelike orbitals are employed, the calculatedHpgr;reproduces experimental and calculated (by configuration interaction) valence state excitation energies, but fails to obtain the diffuse Rydberghyphen;like pihyphen;electron states which interleave the valence spectrum. The addition of a pair of Rydberg molecular orbitals to the valence shell leads to an effective HamiltonianHvwhich accurately describes both valence and Rydberg states simultaneously. These results provide an explanation as to why semiempirical pihyphen;electron theories have required the use of different parameters, particularly resonance integrals, to calculate different properties. Our calculatedHpgr;contains hybrid, exchange, and multicenter twohyphen;electron integrals which are customarily ignored to varying degrees in zero differential overlap (ZDO) approximation based methods. TheHpgr;integrals often differ considerably from their lsquo;lsquo;theoretical,rsquo;rsquo; uncorrelated counterparts, and ourHpgr;calculation provides further insight into these ZDO approximations. A more direct comparison is made with the traditional Pariserndash;Parrndash;PopleHPPPby averaging ourHpgr;to generate a formHmacr;pgr;similar toHPPP. Certain aspects ofHmacr;pgr;are very close quantitatively toHPPP, but there are other strong differences which are easily understood physically. The fullHpgr;with all repulsion integrals and even effective threehyphen;electron interactions can now be used as a testing ground against which new types of semiempirical approximations may be tested.