Active Site Models Computational Considerations

The choice of a theoretical model usually depends on the goals of the study, but sometimes other considerations, such as computer resources available, can also play a role in this selection process. A study of zeolite framework acidity or catalytic activity requires an explicit consideration of the electronic structure of the system, and quantum mechanical (QM) models are best suited for such investigations. Since the high CPU demands greatly limit the size of the systems that can be simulated with quantum mechanical models, the zeolite framework is often represented in these simulations by a cluster that presumably resembles the active site. The cluster approximation has the obvious drawback that influences of the crystal lattice are neglected. 

Given recent developments in computer modeling of chemical reactions, there is considerable interest in attempting to develop a mathematical understanding of enzyme catalysis. The sheer complexity of enzymes means that at present, it is possible to apply a strict quantum mechanical approach to only a limited region, such as the active site, and classical molecular mechanics are used to describe the remainder of the molecule. This combined approach had some success in modeling some aspects of enzyme-catalyzed reactions, such as the importance of particular side chains in the catalytic process.However, a complete mathematical description of enzyme catalysis remains a considerable way off. 

The computational studies on surface chemistry of Co catalysts have offered significant supports to the investigation of FTS on Co catalysts. However, the work is far from decent. As the experimental studies indicated, surface reconstruction and phase transition were certain to take place under practical FTS conditions. The theoretical studies about surface reconstruction and phase transition of Co catalysts, however, are fairly rare. In addition, cluster models were less studied in the previous theoretical work compared to slab models. However, practical catalytic reactions do not always happen as proposed in ideal plane models, nor the active sites distribute homogeneously on the surface. The investigation on cluster models is acting a crucial role in the study of heterogeneous catalysis. Accordingly, more considerations on surface reconstruction, phase transition, and cluster models should be taken into account in future work. 

---