Magnetic cluster model

Ellis D E, Benesh G A and Byrom E 1978 Self-consistent embedded-cluster model for magnetic impurities p -NiAl J. Appl. Phys. 49 1543... 

Fig. 6.17. Calculated hyperfine shifts for cysteine P-CH2 protons of the C04S11 cluster from a magnetic coupling model with 714 = 0 and all other J values equal to 50 cm 1. Numbers refer to the metals to which the cysteines are coordinated (bridging cysteines being identified by two numbers).

The combination of the cluster model approach and modem powerful quantum chemistry techniques can provide useful information about the electronic structure of local phenomena in metal oxides. The theoretical description of the electronic states involved in local optical transitions and magnetic phenomena, for example, in these oxides needs very accurate computational schemes, because of the generally very large differential electron correlation effects. Recently, two very promising methods have become available, that allow to study optical and magnetic phenomena with a high degree of precision. The first one, the Differ-... 

It seems of interest to compare the covalent and ionic cluster models as applied to the same system. Such a comparison was made within the scope of CNDO/2 calculations of some characteristics of surface hydroxyls of Si02 (3magnetic resonance parameters of surface sites (39), and chemisorption (40). Though somewhat different, the results obtained within these models were quite close. Nevertheless, to make a more categorical conclusion, more extensive studies are required. It is likely that in several cases the close-packed oxide structures would be better represented by stoichiometric clusters (e.g., Mg2 02n), although even in these cases it seems appropriate to take into account the Madelung field of the nearest part of the resting lattice. 

It is not possible to give here a complete review of DMol applications, so only a non-systematic selection of applications is mentioned here. Applications to chemical reactions have been studied by Seminario, Grodzicki and Politzer [10]. Buckminster-fullerenes have been studied by various groups [11] including also nonlinear optical properties [8] and the geometrical structure of Cs4 [13]. Cluster model studies of surfaces with adsorbates are reported in [14-17]. Cluster models for point defects in solids, in particular spin density studies of interstitial muon can be found in [18,19]. Spin density studies of molecular magnetic materials are in ref [20]. Polymers have been studied by Ye et al [21]. 

Singularities in E show up for v 0 and with appropriate changes of the parameters are the same as discussed for the magnetic rotator model. The uniqueness of the groundstate implies the cluster decomposition property for correlation functions... 

Important questions of metal adsorption at ionic metal oxides are (i) preferred adsorption sites (ii) strength and nature of metal-support interactions on regular and defect-rich surfaces (iii) charge redistribution between deposits and supports (iv) geometric and electronic structure as well as magnetism of small metal particles and deposition-induced alteration of these features (v) implications for the reactivity. We will also discuss accuracy improvements due to more precise xc functionals as well as more realistic cluster models of oxide supports. 

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