Density functional theory intermolecular interactions, electron

The ab initio methods used by most investigators include Hartree-Fock (FFF) and Density Functional Theory (DFT) [6, 7]. An ab initio method typically uses one of many basis sets for the solution of a particular problem. These basis sets are discussed in considerable detail in references [1] and [8]. DFT is based on the proof that the ground state electronic energy is determined completely by the electron density [9]. Thus, there is a direct relationship between electron density and the energy of a system. DFT calculations are extremely popular, as they provide reliable molecular structures and are considerably faster than FFF methods where correlation corrections (MP2) are included. Although intermolecular interactions in ion-pairs are dominated by dispersion interactions, DFT (B3LYP) theory lacks this term [10-14]. FFowever, DFT theory is quite successful in representing molecular structure, which is usually a primary concern. 

We treat, in this chapter, mainly solid composed of water molecules such as ices and clathrate hydrates, and show recent significant contribution of simulation studies to our understanding of thermodynamic stability of those crystals in conjunction with structural morphology. Simulation technique adopted here is not limited to molecular dynamics (MD) and Monte Carlo (MC) simulations[l] but does include other method such as lattice dynamics. Electronic state as well as nucleus motion can be solved by the density functional theory[2]. Here we focus, however, our attention on the ambient condition where electronic state and character of the chemical bonds of individual molecules remain intact. Thus, we restrict ourselves to the usual simulation with intermolecular interactions given a priori. 

Volkov, A., Koritsanszky, T, and Coppens, P. [2004]. Combination of the exact potential and multipole methods [EP/MM] for evaluation of intermolecular electrostatic interaction enei ies with pseudoatom representation of molecular electron densities, Chem. Phys. Lett 391, pp. 170-175, dol 10.1016/j.cplett.2004.04.097. von Lilienfeld, 0. A., Tavernelli, 1., Rothlisberger, U., and Sebastian , D. [2004]. Optimization of effective atom centered potentials for London dispersion forces in density functional theory, Phys. Rev. Lett 93,15, p. 153004. 

Density functional theory (DPT) is not presently suitable for intermolecular interactions (Tsuzuki and Liithi 2001 van Mourik and Gdanitz 2002). The main reason for this failure of DPT is the highly non-local nature of the dispersion interaction which is present even when charge densities do not overlap. Since most density fimctionals are local or semi-local in the density, that is, they depend on the electron density and gradients of the density, they are imable to account for non-local correlations, and therefore cannot describe the dispersion energy. 

Abstract The evaluation of key properties of materials using quantum mechanics (QM) methods is the aim of this chapter. The use of QM is necessary to calculate properties that depend on electron interactions or electron density polarization. Following the Introduction, which covers computational chemistry notions, some basic concepts concerning the Density Functional Theory (DFT) used in the presented calculations are illustrated, in addition to a brief review of intermolecular interactions. The chapter then reviews the assessment of some fundamental quantities, such as the adsorption energies of gases and hydrogen in nano-porous materials and on metallic surfaces, respectively. Finally, the calculation of hydrogen solubilization in metal alloys will be also presented. 

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