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The new ONIOM (our own n-layered integrated molecular orbital and molecular mechanics) approach has been proposed and shown to be successful in reproducing benchmark calculations and experimental results. ONIOM3, a three-layered version, divides a system into an active part treated at a very high level of ab initio molecular orbital theory like CCSD(T), a semiactive part that includes important electronic contributions and is treated at the HF or MP2 level, and a nonactive part that is handled using force field approaches. The three-layered scheme allows us to study a larger system more accurately than the previously proposed two-layered schemes IMOMO, which can treat a medium size system very accurately, and IMOMM, which can handle a very large system with modest accuracy. This three-layered scheme has been applied to activation barriers for the Diels−Alder reaction of acrolein + isoprene, acrolein + 2-tert-butyl-1,3-butadiene, and ethylene + 1,4-di-tert-butyl-1,3-butadiene. In general, the results for both geometry optimizations and single point energy calculations agree well with benchmark predictions and experimental results. The scheme has also been applied to the transition state for the oxidative addition of H2 to Pt(P(t-Bu)3)2. The activation energy of this 83-atom reaction is predicted to be 14.2 kcal/mol with the ONIOM3(CCSD(T):MP2:MM3) method.

AbstractsA new method is proposed for the analysis of components of molecular interaction energy within the Hartree-Fock approximation. The liartree-Fock molecular orbitals of the isolated molecules are used as the basis for the construction of Fock matrix of the supermolecule. Then certain blocks of this matrix are set to zero subject to specify boundary conditions of the supermolecule molecular orbitals, and the resultant matrix is diagonalized iteratively to obtain the desired energy components. This method can be considered as an extension of our previous method, but has an advantage in the explicit definition of the charge transfer energy, placing it on an equal footing with the exchange and polarization terms. The new method is compared with existing perturbation methods, and is also applied to the energy and electron density decomposition of (H,O),.On propose une nouvelle mkthode pour analyser les composantes de I'energie d'interaction moliculaire dans le cadre de I'approximation Hartree-Fock. Les orbitales moleculaires des molkcules isolkes sont utiliskes comme base pour construire la matrice de Fock de la supermolecule. Certains blocs de cette matrice sont mis egaux a zero selon des conditions aux limites spkcifiques et la matrice rksultante est diagonalisee d'une faqon iterative pour obtenir les composantes cherchkes. Cette mkthode-ci peut itre considerke comme une extension de notre mkthode antkrieure mais elle a un avantage dans la definition explicite de I'energie de transfert de charge, qui est mise ici au mime rang que les termes d'kchange et de polarisation. La nouvelle mkthode est comparke aux methodes de perturbation traditionelles. Elle a dtk appliquee a la decomposition de I'energie et de la densite ilectronique de (H20)2.Es wird eine neue Methode fur die Analyse der Komponenten der molekularen Wechselwirkungsenergie in der Hartree-Fock-Naherung vorgeschlagen. Die Hartree-FockOrbitale der freien Molekiile werden verwendet als Basis fur die Konstruktion der Fock' schen Matrix des Supermolekiils. Gewisse Blocke dieser Matrix werden nach spezifischen Grenzbedingungen gleich Null gesetzt, und die resultierende Matrix wird in einer iterativen Weise diagonalisiert um die gewiinschten Energiekomponenten zu geben. Diese Methode kann als eine Ergazung unserer friiheren Methode betrachtet werden. Sie hat aber einen Vorteil in der expliziten Definition der Ladungsuberfuhrungsenergie, die hier an denselben Niveau wie die Austausch-und Polarisationsglieder hingestellt wird. Die neue Methode wird mit existierenden Storungsmethoden verglichen. Sie wird auf die Zerlegung der Energie und der Elektronendichte von (H20), angewandt.

A new computational scheme integrating ab initio and molecular mechanics descriptions in different parts of the same molecule is presented. In contrast with previous approaches, this method is especially designed to allow the introduction of molecular mechanics corrections in full geometry optimizations concerning problems usually studied through ab initio calculations on model systems. The scheme proposed in this article intends to solve some of the systematic error associated with modeling through the use of molecular mechanics corrections. This method, which does not require any new parameter, evaluates explicitly the energy derivatives with respect to geometrical parameters and therefore has a straightforward application to geometry optimization. Examples of its performance on two simple cases are provided: the equilibrium geometry of cyclopropene and the energy barriers on S , 2 reactions of alkyl chloride systems. Results are in satisfactory agreement with those of full ab initio calculations in both cases. 0 1995 by John Wiley & Sons, Inc.

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The purpose of this paper is 2-fold. First, we present several extensions to the ONIOM(QM:MM) scheme. In its original formulation, the electrostatic interaction between the regions is included at the classical level. Here we present the extension to electronic embedding. We show how the behavior of ONIOM with electronic embedding can be more stable than QM/MM with electronic embedding. We also investigate the link atom correction, which is implicit in ONIOM but not in QM/MM. Second, we demonstrate some of the practical aspects of ONIOM(QM:MM) calculations. Specifically, we show that the potential surface can be discontinuous when there is bond breaking and forming closer than three bonds from the MM region.

Ab initio LCAO–MO–SCF calculation for H2CO···H2O is carried out with a minimal Slater basis set. The most stable conformation has an O···H distance of 1.89 Å with <C=O···H=− 64° and a stabilization energy of 3.5 kcal/mole, about a half of that for H2O···H2O. Nonlinear and π hydrogen bonds, H2CO···2H2O and the O···H–C hydrogen bond in H2O···HCHO, are also studied. An energy decomposition scheme is proposed and applied to H2CO···H2O and H2O···H2O. In the latter the electrostatic energy 8.0 kcal/mole, the exchange repulsion − 9.9 kcal/mole, the polarization and dispersion energy 0.3 kcal/mole, and the delocalization energy 8.2 kcal/mole are in good agreement with Coulson's estimates.

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