Quantum mechanics intermolecular interaction, electronic

Molecular dynamics and Monte Carlo simulations have been used in the last decade to model ionic liquids. [4] Most of these simulations are classical as opp>osed to quantum mechanical the interactions between chemical species are modelled by emparical force fields, hence, the quality of these simulations is strongly dependent on the intermolecular potential employed. It has been proven that it is possible to find force fields accurate enough to describe ILs. [41] Long and large simulations using ab initio are impractical, however, ab initio molecular dynamics (AIMD) are somewhat less computer demanding and provide an accurate picture of ion-ion interactions of the liquid state, by combining electronic structure calculations with conventional Molecular Dynamic (MD) methods. 

In this paper a method [11], which allows for an a priori BSSE removal at the SCF level, is for the first time applied to interaction densities studies. This computational protocol which has been called SCF-MI (Self-Consistent Field for Molecular Interactions) to highlight its relationship to the standard Roothaan equations and its special usefulness in the evaluation of molecular interactions, has recently been successfully used [11-13] for evaluating Eint in a number of intermolecular complexes. Comparison of standard SCF interaction densities with those obtained from the SCF-MI approach should shed light on the effects of BSSE removal. Such effects may then be compared with those deriving from the introduction of Coulomb correlation corrections. To this aim, we adopt a variational perturbative valence bond (VB) approach that uses orbitals derived from the SCF-MI step and thus maintains a BSSE-free picture. Finally, no bias should be introduced in our study by the particular approach chosen to analyze the observed charge density rearrangements. Therefore, not a model but a theory which is firmly rooted in Quantum Mechanics, applied directly to the electron density p and giving quantitative answers, is to be adopted. Bader s Quantum Theory of Atoms in Molecules (QTAM) [14, 15] meets nicely all these requirements. Such a theory has also been recently applied to molecular crystals as a valid tool to rationalize and quantitatively detect crystal field effects on the molecular densities [16-18]. 

The intermolecular forces of adhesion and cohesion can be loosely classified into three categories quantum mechanical forces, pure electrostatic forces, and polarization forces. Quantum mechanical forces give rise both to covalent bonding and to the exchange interactions that balance tile attractive forces when matter is compressed to the point where outer electron orbits interpenetrate. Pure electrostatic interactions include Coulomb forces between charged ions, permanent dipoles, and quadrupoles. Polarization forces arise from the dipole moments induced in atoms and molecules by the electric fields of nearby charges and other permanent and induced dipoles. 

The NMR chemical shift, the most prevalent parameter in NMR spectroscopy, carries a wealth of information regarding the environment and the local electronic structure in the vicinity of the nucleus under study.(i). For example, one normally finds a different chemical shift for the Ca nucleus of each alanine residue in a protein. Ideally, a thorough analysis of the NMR chemical shift can yield information regarding the structure and interactions in the vicinity of the nucleus concerned. To achieve this, a detailed understanding of how geometrical factors and intermolecular interactions influence the chemical shift is crucial. The development and validation of the methods towards this end have combined powerful and efficient ab initio quantum mechanical techniques, which have been... 

The theory for this intermolecular electron transfer reaction can be approached on a microscopic quantum mechanical level, as suggested above, based on a molecular orbital (filled and virtual) approach for both donor (solute) and acceptor (solvent) molecules. If the two sets of molecular orbitals can be in resonance and can physically overlap for a given cluster geometry, then the electron transfer is relatively efficient. In the cases discussed above, a barrier to electron transfer clearly exists, but the overall reaction in certainly exothermic. The barrier must be coupled to a nuclear motion and, thus, Franck-Condon factors for the electron transfer process must be small. This interaction should be modeled by Marcus inverted region electron transfer theory and is well described in the literature (Closs and Miller 1988 Kang et al. 1990 Kim and Hynes 1990a,b Marcus and Sutin 1985 McLendon 1988 Minaga et al. 1991 Sutin 1986). 

Detailed discussion of quantum mechanics (149) is clearly beyond the scope of this review, and its applications to molecular mechanics and modeling will be briefly summarized. Molecular mechanics is based on the laws of classical physics and deals with electronic interactions by highly simplified approximations such as Coulomb s law. All forces operating in intermolecular interactions are essentially electronic in nature. Any effort to quantitate those forces requires detailed information... 

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