A b i n i t i o test of the pairwise additivity assumption of semiempirical electronic structure: Spectator model of correlation contributions

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Abstract
Correlated, ab initio effective valence shell Hamiltonian (Hν) calculations of CH2 are used to test the pairwise additivity assumption of semiempirical electronic structure methods in which it is assumed that the one‐ and two‐center semiempirical integrals in an atomic basis are independent on the molecular environment (except for the one‐electron, one‐center Coulomb integral). Computed ab initio Hν matrix elements are transformed into a molecule independent atomic basis and are analyzed as a function of bond length and bond angle. Large departures from the pairwise additivity assumption emerge for several classes of Hν matrix elements. The computed behavior is well represented by a spectator model for the influence of molecular environment on correlation contributions to individual Hν matrix elements. Tests are also made of the applicability of Mulliken type approximations to Hν integrals.