Applications of ab initio method

In this chapter, we present a brief background account of recent progress in the application of ab initio methods to elucidating the fundamental physicochemical processes that take place at the electrified aqueous metal interface. Our focus is predominantly on model single-crystal surfaces at lower surface coverages. 

The use of semi-empirical or even force field methods, which are generally used for problems of this size, is nevertheless not apt for the description of these systems [3]. On the other hand, the application of ab initio methods requires a considerable computational effort so that attempts to improve the efficiency of computational techniques are well worthwhile. 

For many years however, the chief problem in the application of ab initio methods to many problems of interest... 

In 1995 Oldfield reviewed the applications of ab initio methods to chemical shift structure refinement, with special emphasis on the construction of chemical shift hyper-surfaces 3(y>, yf) for proteins, de Dios further discussed the range of ab initio calculations using NMR chemical shifts, focussing on the construction of computed chemical shift hyper-surfaces for proteins. In a more recent review, Ando et alf presented an overview of the interplay... 

In many cases, simulation methods are used in a complementary manner to experimental studies, with the validity of the calculations assessed by comparing simulated properties (e.g., crystal structure and activation energies) with those determined experimentally. The major factor in determining the reliability of all the simulation methods is the accuracy of the description of the interaction between the ions. The majority of studies of ionically conducting systems have utilized parameterized potentials containing explicit expressions for the various interactions (short-range repulsion. Coulomb, etc.), although recent advances in available computer power have enabled the application of ab initio methods (see Chapter 7). 

The following sections review a selection of characteristic recent applications of ab initio methods to the spectroscopy of ionic impurities. Our main goal is to illustrate the strengths and limitations of existing approaches to treat electron correlation and spin-orbit effects, and we will also emphasise the importance of crystal effects. 

Recently significant progress has been achieved in the theoretical prediction of NMR parameters. The application of ab initio methods allows for a reliable evaluation of the characteristics of hydrograi-bonded and van der Waals complexes. The theory and methods applied to calculations of spin-spin coupling constants are discussed by M. Pecul and J. Sadlq. The audiors provide a nximber of applications including simple complexes, model systems for nucleic acids and proteins, and weak van der Waals systems. 

The application of ab initio methods has allowed to discern between the stepwise [2-1-2] mechanism and the concerted [3-1-2] mechanism, supporting the latter. Nevertheless, in order to study the enantioselectivity computationally, the size of the system to be considered increases significantly. At this point, the number of atoms to be included in theoretical calculations is too large to be described at QM level, and the use of other computational methods is required. 

The application of ab initio methods in the calculation of harmonic force fields of transition metal complexes has been hampered by the size of these systems and the need to employ costly post-Hartree-Fock methods, in which electron correlation is taken into account. Thus, the fruitful symbiosis between ab initio theory and experiment, to determine empirically scaled quantum mechanical force fields, has been virtually absent in studies of transition metal complexes. 

In this chapter, we provide a succinct review of some of the advances in the development and application of ab initio methods toward understanding the intrinsic reactivity of the metal and the influence of the reactive site and its environment. We draw predominantly from some of our own recent efforts. More specifically we describe (a) the chemistry of the aqueous-phase on transition metal surfaces and its influence on the kinetics and thermodynamics within example reaction mechanisms, and (b) computational models of the electrode interface that are able to account for a referenced and tunable surface potential and the role of the surface potential in controlling electro-catalytic reactions. These properties are discussed in detail for an example reaction of importance to fuel cell electrocatalysis methanol dehydrogenation over platinum(ll 1) interfaces [24,25]. 

It is now established (Hehre et al. 1986) that the ab initio quantum mechanical approaches are very successful in providing reliable values of proton affinities and gas phase basicities for small molecules. However, due to the reason of heavy computational expenses, application of ab initio methods for the estimation of proton affinities is stiU impractical for larger molecules (Labanowskiy et al. 2011). It is also recognized (Ozment et al. 1992) that the popular semi empirical methods such as AMI, MNDO and PM3 are not consistently reliable in calculating proton... 

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