The Full Optimized Reaction Space model for quantum chemical reaction calculations. Definition, applications, and IntraAtomic Correlation Correction extension

The Full Optimized Reaction Space (FORS) model is used for the theoretical calculation of molecular potential energy surfaces involved in chemical reactions. FORS calculations on diatomics serve to demonstrate the model, to illustrate its calculational methodologies, and to assess its quantitative reliability. In addition, the FORS model permits the interpretation of molecular wavefunctions, due to the several choices of molecular orbitals from which the wavefunctions can be formed. In particular, localized orbitals posessing highly atomic character can be isolated from the molecular wavefunctions according to a projection criterion;The FORS model is applied to two polyatomic reactions: the dihydrogen exchange between ethane and ethylene, and the formation and dissolution of dioxirane and dioxymethane. The former reaction is found to possess a high barrier, in spite of its symmetry allowed nature. The latter reaction involves significant configuration mixing as methylene and oxygen react to form, successively, dioxirane, dioxymethane, and hydrogen and carbon dioxide;Finally, FORS wavefunctions can be expressed in terms of abasis of antisymmetrized products of atomic state functions, usingthe predominantly atomic projected localized orbitals. The atomsin molecules analysis permits the incorporation of data fromatomic spectra into the molecular Hamiltonian to achieve theIntraAtomic Correlation Correction (IACC). The IACC scheme is;illustrated for a few small diatomics, and is shown to yield more accurate results than the uncorrected FORS wavefunctions;(\u271)USDOE Report IS-T-1038. This work was performed under Contract No. W-7405-eng-82 with the U.S. Department of Energy