Abstract
A previously proposed semiempirical method has been developed for calculating the molecular electron structures and geometries, which explicitly incorporates AOnonorthogonality. The integrals are parametrized in an atomic basis, and then a transfer is made to an orthogonal basis, in which the zero differential overlap approximation is used. The electron-repulsion integral matrix Г is transformed by a method in which one uses only the Coulomb part. The performance has been evaluated from the number of elementary operations, which is proportional to N3, where N is the basis size. The method is implemented as two schemes that differ in the method of specifying Г in the atomic basis: λ-CNDO and λ-NDDO. Calculations are presented on hydrocarbons by means of the λ-CNDO scheme, which enables one to calculate the heats of formation and the geometry with an accuracy comparable with that in MINDO/3. The performance is higher in calculations on transitional states, as is demonstrated for the model of nucleophilic substitution (CH5 − anion). The λ-NDDO scheme has been examined at the level of electron-interaction integral transformation and has considerable advantages over the λ-CNDO one.
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Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 23, No. 4, pp. 385–394, July–August, 1987.
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Shamov, A.G., Tikhomirov, V.A. & Bazilevskii, M.V. New method of incorporating nonhorizontal AO in semiempirical quantum-chemical calculations. Theor Exp Chem 23, 359–367 (1988). https://doi.org/10.1007/BF00536351
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DOI: https://doi.org/10.1007/BF00536351