Ab initio computation of semiempirical π-electron methods. II. Transferability of ℋν parameters between ethylene, trans-butadiene, and cyclobutadiene

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Abstract
A series of systematic approximations is applied within the correlated, size‐extensive, ab initio effective valence shell Hamiltonian (Hν) formalism to test such basic assumptions of π‐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 π‐electron valence orbitals that are constructed from a set of localized, transferable pπ functions and the neglect of three‐ and four‐body Hν parameters. Our approximate ab initio Hν explicitly contains correlation contributions to all its parameters, and some fundamental differences exist between the ab initio Hν and the standard semiempirical π‐electron methods. Correlation contributions are shown to be essential in justifying the ZDO approximation, but some violations of this approximation appear. Transferability of Hν parameters is often excellent between ethylene, trans‐butadiene, and cyclobutadiene provided that the local geometries are made as similar as possible. The Hν parameters with the largest correlation contributions display the greatest sensitivity to molecular environment, while orbital orthogonalization provides another mechanism inhibiting exact transferability.