Molecular dynamics technique ab initio

In order to study a system, one first has to assume a model interaction potential between the particles that are defined as the constituents of the fluid under investigation. Such a modelization is necessary if it is desired not to perform a quantum mechanical description of the system at the level of a first principle Hamiltonian composed of elementary forces. In the latter case, the ab initio molecular dynamics technique, developed by Car and Parrinello [1, 2], was revealed to be a powerful investigation tool that was adopted by many authors the last two decades. 

Among the numerous theoretical approaches applied to the zeolite reactivity problem, we focus our attention mainly on calculations, which use recently developed ab initio molecular dynamics techniques. After a very brief overview of the main features of this methodology, we discuss some applications taken from the modeling of zeolite chemistry the characterization of the catalytic sites, the protonation of a water molecule and the mechanism of the protolytic reactions of alkanes. 

T.A. Arias, M.C. Payne, and J.D. Joannopoulos, Ab initio molecular-dynamics techniques extended to large-length-scale systems, Phys. Rev. B Condens. Matter, 45 (1992), 1538-1549. 

The ab initio molecular dynamics technique provides a powerful method in studying the properties of chemical systems under varying thermodynamic conditions without having to employ any empirical interaction potentials. In this chapter, a brief review has been made on our recent studies on water dynamics by using this method combined with a time series analysis. We have discussed the frequency-structure correlations of water molecules in both supercritical and normal water. Our calculations reveal that hydrogen bonds still persist to some extent in the supercritical state. However, the quantitative details of hydrogen bonding depend on the density. At... 

So-called ab initio molecular dynamics techniques in which the DFT (usually just in its LDA approximation) is combined with a classical mechanical treatment of the nuclear (ion) motion have been a very popular way of studying condensed matter, i.e., the dynamics of liquids and solids. These techniques may, for instance, be used to study dynamic processes such as binding and atom diffusion [240] at surfaces, and in principle also reactivity at surfaces, without resorting to the usual procedure, namely that of determining the potential energy surface first and then doing the nuclear dynamics afterwards. 

In this chapter we shall consider four important problems in molecular n iudelling. First, v discuss the problem of calculating free energies. We then consider continuum solve models, which enable the effects of the solvent to be incorporated into a calculation witho requiring the solvent molecules to be represented explicitly. Third, we shall consider the simi lation of chemical reactions, including the important technique of ab initio molecular dynamic Finally, we consider how to study the nature of defects in solid-state materials. 

Fig. 11.38 Lag ejfects in ab initio molecular dynamics. (Figure redrawn from Payne MC, M P Teter, D C Allan, R A Arias and D ] Joannopoidos 1992. Iterative Minimisaticm Techniques for Ab Initio Total-Energy Calculations Molecular Dynamics and Conjugate Gradients. Reviews of Modern Physics 64 1045-1097.)...

A successful tool to describe and interpret experimental findings of liquids is to perform ab initio molecular dynamics (MD) simulations for the particular systems. We performed such simulations for 5 different compositions of NaSn - ranging from 20% to 80% of sodium - applying the Car-Parrinello technique [5]. 

Figure 27. Defective structure of solid trifluoromethane-sulfonic acid hydrate (CF3S0sH H20)4 found using ab initio molecular dynamics (AIMD see Section 2.2.3 for a description of the technique), showing two hydronium ions hydro-gen-bonded to sulfonate groups (as found in the perfect structure) but, more importantly, two shared protons (one between two sulfonate groups and the other as part of a Zundel ion see text). Note that the energy of the defective structure is only --30 kj/mol higher than that of the perfect structure.

A proposal for the comprehensive study of chemical processes in a variety of important condensed-phase systems using modern theoretical methodology has been presented. The primary goals of the research are to provide microscopic information on the mechanisms and structural and dynamical properties of the chemical systems proposed for investigation, to test the applicability of modern ab initio molecular dynamics (MD) by comparison with experiment, and to develop and apply novel ab initio MD techniques in simulating complex chemical systems. The proposed research will contribute to the forefront of modern theoretical chemistry and address a number of important technological issues. The PI has carefully attempted to demonstrate his knowledge, ability, and resources to carry out the proposed research projects. 

We have presented nonadiabatic ab initio molecular dynamics simulations of the photophysical properties of a variety of nucleobases and base pairs. In addition to the canonical tautomers a number of rare tautomers have been investigated. Moreover, effects of substitution and solvation have been studied in detail. The simulations of nonradiative decay in aqueous solution, in particular, demonstrate the strength of the na-AIMD technique employed here as it permits the treatment of solute and solvent on an equal footing. Condensed phase calculations can be directly compared with those in the gas phase because the same computational setup can be used. 

This article is organized as follows. In Section 2 ab initio molecular dynamics methods are described. Specifically, in Section 2.1 we discuss the extended Lagrangian atom-centered density matrix (ADMP) technique for simultaneous dynamics of electrons and nuclei in large clusters, and in Section 2.2 we discuss the quantum wavepacket ab initio molecular dynamics (QWAIMD) method. Simulations conducted and new insights obtained from using these approaches are discussed in Section 3 and the concluding remarks are given in Section 4. 

The next section gives a brief overview of the main computational techniques currently applied to catalytic problems. These techniques include ab initio electronic structure calculations, (ab initio) molecular dynamics, and Monte Carlo methods. The next three sections are devoted to particular applications of these techniques to catalytic and electrocatalytic issues. We focus on the interaction of CO and hydrogen with metal and alloy surfaces, both from quantum-chemical and statistical-mechanical points of view, as these processes play an important role in fuel-cell catalysis. We also demonstrate the role of the solvent in electrocatalytic bondbreaking reactions, using molecular dynamics simulations as well as extensive electronic structure and ab initio molecular dynamics calculations. Monte Carlo simulations illustrate the importance of lateral interactions, mixing, and surface diffusion in obtaining a correct kinetic description of catalytic processes. Finally, we summarize the main conclusions and give an outlook of the role of computational chemistry in catalysis and electrocatalysis. 

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