Abinitiocomputation of semiempirical pgr;hyphen;electron methods. III. The benzene molecule, the zerohyphen;differentialhyphen;overlap approximation, and the transferability of parameters

摘要

The correlated, sizehyphen;extensiveabinitioeffective valence shell Hamiltonian (Hngr;) method is used to compute the true pgr;hyphen;electron Hamiltonian of benzene and to use it for testing various assumptions of the Pariserndash;Parrndash;Pople (PPP) semiempirical electronic structure method. On one hand, theabinitioHngr;method enables computing the low lying valencelike vertical excitation energies and ionization potentials. For example, the calculatedHngr;excitation energies deviate from experiment on average by only 0.28 eV, which compares well with similar correlatedabinitiocalculations. More remarkably, however, theHngr;method can reproduce the vertical excitation energies on average to 0.35 eV simply by employing valence orbitals constructed from symmetry adapted linear combinations of carbon atomppgr;orbitals. Thus theHngr;calculations demonstrate that accurate benzene excitation energies can be obtained using a common set of valence orbitals for every valencelike state and, in fact, for every conjugated hydrocarbon! In addition, the true, correlated benzene pgr;hyphen;electron effective integrals areabinitiocounterparts of semiempirical parameters and transfer extremely well to other small molecules such as ethylene and cyclohyphen;butadiene. The degree of transferability is improved when increasing the size of the primitiveabinitiobasis set. An analysis of theHngr;effective integrals demonstrates that the true PPP model can indeed neglect a portion of onehyphen; and twohyphen;electron parameters which are assumed to be small by the zero differential overlap (ZDO) approximation, since the correlation contributions render some effective integrals negligible. On the other hand, some typically neglected parameters, such as twohyphen;electron, threehyphen;center interactions, are computed as too large to just neglect. Likewise, the exact pgr;hyphen;electronHngr;contains threehyphen;electron manyhyphen;body interactions which are large (greater than 0.1 eV) and thus cannot be explicitly neglected in PPP models.