Energetics using the single point IMOMO (integrated molecular orbital+molecular orbital) calculations: Choices of computational levels and model system

Abstract

The integrated MO+MO (IMOMO) method, recently proposed for geometry optimization, is tested for accurate single point calculations. The principle idea of the IMOMO method is to reproduce results of a high level MO calculation for a large ‘‘real’’ system by dividing it into a small ‘‘model’’ system and the rest and applying different levels of MO theory for the two parts. Test examples are the activation barrier of the SN2 reaction of Cl−+alkyl chlorides, the C=C double bond dissociation of olefins and the energy of reaction for epoxidation of benzene. The effects of basis set and method in the lower level calculation as well as the effects of the choice of model system are investigated in detail. The IMOMO method gives an approximation to the high level MO energetics on the real system, in most cases with very small errors, with a small additional cost over the low level calculation. For instance, when the MP2 (Mo/ller–Plesset second-order perturbation) method is used as the lower level method, the IMOMO method reproduces the results of very high level MO method within 2 kcal/mol, with less than 50% of additional computer time, for the first two test examples. When the HF (Hartree–Fock) method is used as the lower level method, it is less accurate and depends more on the choice of model system, though the improvement over the HF energy is still very significant. Thus the IMOMO single point calculation provides a method for obtaining reliable local energetics such as bond energies and activation barriers for a large molecular system.

Mechanisms of and Effect of Coadsorption on Water Dissociation on an Oxygen Vacancy of the MgO(100) Surface

“…[49,50] The IMOMO method allows interactions www.chemeurj.org in a subregion of the system to be treated at a more accurate level of theory and thus provides a cost-effective method for improving energetic properties in our previous studies of adsorption on metal oxide surfaces. [42,47,48] In this case, a smaller quantum cluster consisting of the adsorbed water and nine atoms surrounding the vacancy was selected as a model system, and was treated at the CCSD level with the above mixed basis set.…”

Mechanism of Cysteine Oxidation by a Hydroxyl Radical: A Theoretical Study

“…As result the Gibbs free energies of RC and TS1 are almost equal suggesting that the free energy surface is still more flat than that for the methanethiol reaction. However, these Gibbs free energy variations are not so important to modify the reaction kinetics: the RC and A states are still fast dissociating states and the reaction rate constant can be still calculated by applying Equation (14). This gives for k TST a value of 2.2 10 10 mol À1 dm 3 s…”

“…This hybrid treatment was carried out by means of the integrated molecular orbital + molecular orbital (IMOMO) method of Svensson et al implemented in Gaussian03. [14] Once identified on the scan trajectories, the molecular intermediates were further optimized without constraints at the UMP2/6-311 + GA C H T U N G T R E N N U N G (d,p) level of theory except for the adduct which was optimized at the IMOMO(QCISD:UMP2) level of theory. In the present case using different methods to compute geometries is not inconsistent because the two methods gave practically the same results for all structures but the adduct.…”

The Rate‐Determining Step in the Rhodium–Xantphos‐Catalysed Hydroformylation of 1‐Octene

“…Kinetic isotope effects were calculated by using statistical mechanics employing harmonic vibrational frequencies obtained from frequency calculations. [32] The initial energy analysis of the possible isomers (see Supporting Information) was performed by means of the more computationally affordable hybrid quantum mechanics/molecular mechanics (QM/MM) calculations, by using the ONIOM method [38] as implemented in the GAUSSIAN 98 series of programs. [32] The QM region of the catalysts was [RhH(CO)A C H T U N G T R E N N U N G (PH 3 ) 2 ], whereas the substrate ethene was fully in the QM region.…”