Interatomic interaction forces Molecular interactions

Natural water comprises numerous atoms connected by forces of interatomic and inter-molecular interaction. Values of the chemical potential of each individual compound depends on many parameters, in particular on pressure, temperatme and the presence of other compounds. Especially complex is its dependence on the composition of the solution. For this reason, as opposed to many other intensive parameters, chemical potential cannot be meastued directly in absolute. 

Both of the above approaches rely in most cases on classical ideas that picture the atoms and molecules in the system interacting via ordinary electrical and steric forces. These interactions between the species are expressed in terms of force fields, i.e., sets of mathematical equations that describe the attractions and repulsions between the atomic charges, the forces needed to stretch or compress the chemical bonds, repulsions between the atoms due to then-excluded volumes, etc. A variety of different force fields have been developed by different workers to represent the forces present in chemical systems, and although these differ in their details, they generally tend to include the same aspects of the molecular interactions. Some are directed more specifically at the forces important for, say, protein structure, while others focus more on features important in liquids. With time more and more sophisticated force fields are continually being introduced to include additional aspects of the interatomic interactions, e.g., polarizations of the atomic charge clouds and more subtle effects associated with quantum chemical effects. Naturally, inclusion of these additional features requires greater computational effort, so that a compromise between sophistication and practicality is required. 

H-bonding is an important, but not the sole, interatomic interaction. Thus, total energy is usually calculated as the sum of steric, electrostatic, H-bonding and other components of interatomic interactions. A similar situation holds with QSAR studies of any property (activity) where H-bond parameters are used in combination with other descriptors. For example, five molecular descriptors are applied in the solvation equation of Kamlet-Taft-Abraham excess of molecular refraction (Rj), which models dispersion force interactions arising from the polarizability of n- and n-electrons the solute polarity/polarizability (ir ) due to solute-solvent interactions between bond dipoles and induced dipoles overall or summation H-bond acidity (2a ) overall or summation H-bond basicity (2(3 ) and McGowan volume (VJ [53] ... 

Figure 1 An example of the pair distribution function g(r), the potential of mean force w(r), and the true interatomic potential u(r) based on our molecular dynamic simulations with a simple monatomic system. The only interatomic interactions in the system are the Lennard-Jones format potential.

Surface forces acting between mesoscopic or macroscopic bodies can be described by summing up pairwise interatomic (intermolecular) forces for each participating atomic (molecular) pairs within the interacting bodies. Semiempirical... 

It is weU known that many materials, whether they have originally ionic, non-ionic, or molecular lattice stmctures, are transformed into the metallic state by the application of sufficiently high pressure, and indeed this can be expected to be tme of aU materials. Even quite modest increases in pressure can affect interatomic distances, spectral transitions, formal oxidation states, and many other phenomenological parameters, e.g. can increase the coordination number. Various attempts have been made in an effort to estabhsh relationships between pressure and these phenomenological parameters but none of them accounts satisfactorily for all of the observed features. This is almost certainly because of the absence, up to now, of a model which is capable of interpreting the facts without concerning itself with too detailed an interpretation of the binding forces. However, it will be shown here, after a brief survey of the present situation, that the functional approach seems to successfully provide such a model based as it is on an electron-pair donor-acceptor model of molecular interactions. 

The classical equations of motion used in molecular mechanics (MM) are only slightly more difficult to solve than simple additive bond energy equations hence MM calculations are fast and not very demanding of computer resources. In molecular mechanics, one determines the structure of a molecule from a knowledge of the force field, a collection of empirical force constants governing, in principle, all classical mechanical interatomic interactions within the molecule. In practice, it is not feasible for a parameter set to include all possible interactions within a complicated molecule. One hopes that all significant interactions have been included in the force field. 

MD simulations of electrolytes for lithium batteries retain the atomistic representation of the electrolyte molecules but do not treat electrons explicitly. Instead the influence of electrons on intermolecular interactions is subsumed into the description of the interatomic interactions that constitute the atomistic potential or force field. The interatomic potential used in MD simulations is made up of dispersion/ repulsion terms. Coulomb interactions described by partial atomic charges, and in some cases, dipole polarizability described by atom-based polarizabilities. The importance of explicit inclusion of polarization effects is considered below. In the most accurate force fields, interatomic potentials are informed by high-level QC calculations. Specifically, QC calculations provide molecular geometries, conformational energetic, binding energies, electrostatic potential distributions, and dipole polarizabilities that can be used to parameterize atomic force fields. 

The interpretation of molecular orbital calculations on conformational isomers is not as straightforward as for molecular mechanics methods. Because MO calculations treat all of the bonding forces of the molecule, the difference between two conformations represents only a small part of the total energy. Furthermore, unlike the molecular mechanics model in which energies are assigned to specific interatomic interactions, the energy of a specific molecular orbital may encompass contributions from a number of intermolecular interactions. Thus, the identification of the structural features responsible for the energy difference between two conformers may be very difficult. 

The main question when we started to simulate SPE by molecular dynamics was the choice of the interatomic potential. Crystalline silicon is one of the most studied materials, and numerous formulations of the interaction forces between Si atoms exist [2,3, 16-24]. These different developments and parametrisation were made because, due to the assumptions used as a basis of their formulation, empirical potential can... 

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