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      EXTENDED HÜCKEL ORBITAL FORCES

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          Abstract

          The forces acting on individual atoms in a molecule, arising from the distorting effect on the molecular framework of the electrons in the different populated molecular orbitals, are calculated within a simple Extended Hückel formalism. The method, which is strongly dependent on the properties of overlap integrals, is applied to the study of several processes and reactions of chemical interest, such as conformational stability of molecules, torsional stability for rotations around a single bond of some simple molecules and the connection between the forces acting upon atoms and the rules derived from conservation of orbital symmetry in concerted reaction. The results show good agreement with the experimental evidence and the known symmetry rules

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          Basis set convergence of the coupled-cluster correction, δ(MP2)(CCSD(T)): best practices for benchmarking non-covalent interactions and the attendant revision of the S22, NBC10, HBC6, and HSG databases.

          In benchmark-quality studies of non-covalent interactions, it is common to estimate interaction energies at the complete basis set (CBS) coupled-cluster through perturbative triples [CCSD(T)] level of theory by adding to CBS second-order perturbation theory (MP2) a "coupled-cluster correction," δ(MP2)(CCSD(T)), evaluated in a modest basis set. This work illustrates that commonly used basis sets such as 6-31G*(0.25) can yield large, even wrongly signed, errors for δ(MP2)(CCSD(T)) that vary significantly by binding motif. Double-ζ basis sets show more reliable results when used with explicitly correlated methods to form a δ(MP2-F12)(CCSD(T(*))-F12) correction, yielding a mean absolute deviation of 0.11 kcal mol(-1) for the S22 test set. Examining the coupled-cluster correction for basis sets up to sextuple-ζ in quality reveals that δ(MP2)(CCSD(T)) converges monotonically only beyond a turning point at triple-ζ or quadruple-ζ quality. In consequence, CBS extrapolation of δ(MP2)(CCSD(T)) corrections before the turning point, generally CBS (aug-cc-pVDZ,aug-cc-pVTZ), are found to be unreliable and often inferior to aug-cc-pVTZ alone, especially for hydrogen-bonding systems. Using the findings of this paper, we revise some recent benchmarks for non-covalent interactions, namely the S22, NBC10, HBC6, and HSG test sets. The maximum differences in the revised benchmarks are 0.080, 0.060, 0.257, and 0.102 kcal mol(-1), respectively.
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            J. Am. Chem. Soc.

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              J. Chem. Soc.

               L. Orgel,  D Craig,  A Maccoll (1966)
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                Author and article information

                Contributors
                Role: ND
                Role: ND
                Role: ND
                Journal
                jcchems
                Journal of the Chilean Chemical Society
                J. Chil. Chem. Soc.
                Sociedad Chilena de Química (Concepción )
                0717-9707
                December 2003
                : 48
                : 4
                : 105-113
                Affiliations
                [1 ] Pontificia Universidad Católica de Chile Chile
                [2 ] Universidad de Chile Chile
                Article
                S0717-97072003000400017
                10.4067/S0717-97072003000400017

                http://creativecommons.org/licenses/by/4.0/

                Product
                Product Information: SciELO Chile
                Categories
                CHEMISTRY, MULTIDISCIPLINARY

                General chemistry

                Orbital Forces, Orbital Interactions

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