AB-INITIO COMPUTATION OF SEMIEMPIRICAL PI-ELECTRON METHODS .2. TRANSFERABILITY OF H-NU PARAMETERS BETWEEN ETHYLENE, TRANS-BUTADIENE, AND CYCLOBUTADIENE

A series of systematic approximations is applied within the correlated , size-extensive, ab initio effective valence shell Hamiltonian (H-nu) formalism to test such basic assumptions of pi-electron theory as the zero-differential-overlap (ZDO) approximation and the transferability of semiempirical matrix elements (called parameters). The systematic approximations (tested in paper I) involve the use of pi-electron vale nce orbitals that are constructed from a set of localized, transferabl e p(pi) functions and the neglect of three- and four-body H-nu paramet ers. Our approximate ab initio H-nu explicitly contains correlation co ntributions to all its parameters, and some fundamental differences ex ist between the ab initio H-nu and the standard semiempirical pi-elect ron methods. Correlation contributions art shown to be essential in ju stifying the ZDO approximation, but some violations of this approximat ion appear. Transferability of H-nu parameters is often excellent betw een ethylene, trans-butadiene, and cyclobutadiene provided that the lo cal geometries are made as similar as possible. The H-nu parameters wi th the largest correlation contributions display the greatest sensitiv ity to molecular environment, while orbital orthogonalization provides another mechanism inhibiting exact transferability.