Embedded cluster model computation

Computational studies of dc Carolis et have combined MD simulations of both Ca-doped and undoped CeOz with QM electronic structure calculations of embedded cluster models built from the MD structures. Several interesting aspects were... 

Instead of this type of model, we suggest utilization of TST in the recently developed variational versions (Lauderdale and Truhlar 1985,1986 Truhlar et al. 1986 Truong et al. 1989b). These can be either microcanonical (E-,) or canonical (T) forms. These also include multidimensional tunneling via least-action techniques as well as surface atom motion via embedded cluster models. They do require a PES, but in our opinion, it is preferable to at least indicate a PES rather than make a multitude of assumptions about the dynamics. Because of the speed of the VTST methods, large computing facilities would not be necessary, and a well-documented program exists for these calculations, which should be available by the time of publication of this review (from QCPE at the University of Indiana). 

L. Seijo and Z. Barandiaran. The ab initio model potential method A common strategy for effective core potential and embedded cluster calculations. In J. Leszczynski, (ed), Computational chemistry Reviews of Current Trends, 4, pp. 55-152, World Scientific, Singapore, 1999. 

The overwhelming majority of the theoretical studies were performed on cluster models of the catalytic site, hi spite of the fact that the role of space confinement and the secondary interactions with the framework atoms is well-known, there are only a few electronic structure calculations on lattice models involving hydrocarbons, using either periodic DFT calculations, or embedding methods. In this brief account of the subject we attempt to overview some of the recent computational results of the literature and present some new data obtained from ab initio DFT pseudopotential plane wave calculations on Cl - C4 alkanes in the chabazite framework. 

A crucial feature of the metal/oxide interface is that it combines the extended nature of the support with the limited size of the supported metal particles - a feature that is common to the study of defects in solids. Such systems pose special problems for realistic modelling. The requirements of defining computationally tractable, as well as accurate models are of particular importance. Three different approaches are common, namely bare clusters, embedded clusters and periodic slab models. All three are associated with approximations, and the best choice must be defined by the correct compromise between cost and accuracy. 

Quantum Chem. 1988, S22, in press (RSGK),(35))> embedding the cluster would be yet another possibility (3. All these improvements entail significantly larger computational efforts. For ab initio methods large cluster models have been feasible only by introducing some effective core approximation. 

Three types of models were repeatedly used in the theoretical description of zeolites (i) periodic models, (ii) cluster models, and (iii) combined (named also hybrid or embedded) models. Cluster models have dominated in the zeolite science in the eighties and ninctics.[2] However, with ever improving computational hardware and with a progress in the software development (various embedding schemes applicable for zeolites, e. g., Refs. [24,25] and... 

Artifacts due to immediate contacts of frontier cluster anions with positive PCs of the surrounding can be reduced if one substitutes these PCs with bare model potential cations (pseudopotential, PP), taken without basis functions. This helps to restore the repulsion between electron shells of anions of the cluster and cations of the environment, an interaction that is missing in a cluster embedding based in merely a PC array. Less efficient from the computational viewpoint is the brute force alternative where positive PCs at the cluster borders are replaced by all-electron (AE) cations in the latter case, one is forced to use formal (nominal) charges of the environmental ions and, as a rule, the QM cluster model is non-stoichiometric. 

For touchstone complexes of metal atoms on MgO(OOl), results of DF cluster [96,167] and periodic [191-193] calculations agree. In particular, (i) adsorption sites on-top of O are preferred, (ii) bonding can be rationalized by a polarization of metal atoms in the electrostatic field of the support with modest covalent and only very minor ionic contributions, (iii) adsorption energies are close in these model cluster and periodic slab computational approaches, provided the same xc functional is used. Embedded cluster GGA BP86 calculations on representative systems of Pd and Pt atoms on MgO(OOl) yielded binding energies of 135 kJ/mol [73] and 232 kJ/mol [170], respectively. 

Our earlier computational investigation dealt with model clusters Cu4, Ag4, Ni4 and Pd4 on MgO(OOl) considered in C4V symmetry [172]. To complement that work we studied the adsorption properties of Ag and Pd tetramers (C2V symmetry constraints) deposited at the regular (001) surface of MgO [173]. The calculations of cluster models embedded in arrays of PCs and AE Mg cations were carried out with the BP86 GGA at the scalar relativistic level. 

Pisani C, Ricca F (1980) Embedded versus non-embedded-cluster treatment of chemisorbed systems— model computation. Surf Sci 92 481-488... 

We have also described the results of theoretical studies of the vibrational spectra of hexagonal and cubic ice in the O—H and O—D stretching regions. They include simulation of IR and Raman spectra, the effects of isotopic dilution on the IR and polarized Raman spectra, and computational modeling of the observed influence of dilution on the properties of vibrationally excited states. In the crystalline isotopom-ers the properties of the spectra and the vibrationally excited states are determined by a complex interplay between the size distributions of the embedded clusters and the inter- and intramolecular couplings. Quantum and MD calculations permit calculation of the spectra of amorphous ice and spectra of water in ionic shells. 

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