ONIOM approach

A few points should be clarified. First, the ONIOM approach can be viewed as perturbational, i.e., the whole system is first treated at lower level (molecular mechanics, MM) of theory but since we expect this level to be inadequate for the... 

Sato presents an alternative method to both continuum solvation models and hybrid QM/MM or ONIOM approaches. This is represented by the reference interaction site model (RISM) formalism when combined to a QM description of the solute to give the RISM-SCF theory. 

The ONIOM approach (our Own N-layer Integrated molecular Orbital molecular Mechanics) developed by the Morokuma group takes an alternative approach to hybrid QM/MM. The distinct advantages of the ONIOM approach are that... 

Figure 1.6 Schematic of a three-layer ONIOM approach. A low level method (Mg) is...

The basic idea behind the ONIOM approach can be explained most easily when it is considered as an extrapolation scheme in 2D space spanned by the size of the system on one axis and the level of theory on the other axis. In the two-layered ONIOM method, the total energy of the system is obtained from three independent calculations as shown in Eq. 1 ... 

The ONIOM approach provides an extrapolated energy (Eoniom) for a system partitioned in this manner. 

Fig. 6.2 Schematic overview of the linear-scaling ONIOM approach chosen for protein optimization...

Figure 3. Schematic representation of two-layer ONIOM approach...

Fig. 2.3 Concept of the two-layer ONIOM approach (ONIOM2) used for the QM/MM coupling in this work, connecting a high-level method - in our case DFTB - with a low-level method, in our case a force field approach.

The ONIOM approach assumes that entire layer is treated at low level of theory and the main core is considered at a high level of theory, usually DFT or wave-function-based methods. Through the development of whole above mentioned methods, there were numerous works about the conformational analysis of short peptides using theoretical calculations. In the initial phase of the study, the researchers perfected optimization methods in order to build the most real system. According to Renner [180] for the best complementarity of theoretical and experimental result, the combination of... 

It should be noted that since the optimizations were made with ONIOM approach computing most of the aromatic rings at HF/3-21 level of theory, the important weak intramolecular interactions could be neglected in this study. 

As the other steps in cross-coupling reactions, the rate of the reductive elimination step is also influenced by the steric and electronic effects induced by the ancillary ligands bound to the metal center. Consequently, the influence of the steric and electronic properties of these ligands on this step has been theoretically investigated by several groups. Ananikov, Musaev and Morokuma analyzed by means of the ONIOM approach the C-C coupling from the complexes c -[Pd(R)2(L) ] (R = Me, Ph, vinyl, ethynyl L = PPhs, PCys, PMes and PH3 n = 1,2) [116]. According... 

Rega et al. [104] have proposed the QM/MM approach ADMP/ONIOM which implements the atom-centred density matrix propagation within the QM/MM ONIOM scheme of Maseras and Morokuma [105]. The ONIOM approach consists of separating the system in parts described at different levels of theory, for example a model part treated at Hartree-Fock or DFT level (QM), embedded in surroundings that will be treated at a molecular mechanics level (MM). The real system is composed of the model system and of the surrounding system, and the ONIOM energy is ... 

The three- and four-layered schemes will be examined for two Diels-Alder reactions. The addition of acrolein to 2-t-butyl-1,3-butadiene will be examined with the threelayered ONIOM approach where the model will be taken as ethylene + butadiene, the intermediate model as acrolein 4-isoprene, and the real system as acrolein-f 2-l-butyl-1,3-butadiene. The three ab initio levels of theory will be G2MS CCSD(T)/6-31 G(d) -f MP2/6-311+G(2df,2p) -MP2/6-31G(d), MP4(SDQ), and MP2. This three-layered approach will be extended to a four-layered method by replacing the nine r-butyl hydrogens with methyl groups. This becomes the real system, acrolein -f 2-(trimethyl)-r-butyl-1,3-butadiene, with acrolein-f 2- -butyl-1,3-butadiene as the intermediate large model (int.L.model), acrolein -f isoprene as the intermediate small model (int.S.model), and ethylene-f butadiene as the small model (S.model). The four levels of theory are G2MS, MP4(SDQ), MP2, and HF. The geometry of the transition state and reactants have been optimized... 

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