Charge distribution, intermolecular interaction calculations

In this expression, the dipole dipole interactions are included in the electrostatic term rather than in the van der Waals interactions as in Eq. (9.43). Of the four contributions, the electrostatic energy can be derived directly from the charge distribution. As discussed in section 9.2, information on the nonelectrostatic terms can be deduced indirectly from the charge density. The polarizability a, which occurs in the expressions for the Debye and dispersion terms of Eqs. (9.41) and (9.42), can be expressed as a functional of the density (Matsuzawa and Dixon 1994), and also obtained from the quadrupole moments of the experimental charge density distribution (see section 12.3.2). However, most frequently, empirical atom-atom pair potential functions like Eqs. (9.45) and (9.46) are used in the calculation of the nonelectrostatic contributions to the intermolecular interactions. 

The potential outside the charge distribution and due to it is simply related to the moments, as is the interaction energy when an external field is applied.14 The multipole moments are thus very useful quantities and have been extensively applied in the theory of intermolecular forces, particularly at long range where the electrostatic contribution to the interaction may be expanded in moments. Their values are related to the symmetry of the system thus, for instance, a plane of symmetry indicates that the component of n perpendicular to it must be zero. Such multipoles are worth calculating in their own right. 

Electrostatic term for each component, an ab initio calculation is carried out to obtain the wavefunction. The charge distribution is then written as a sum of multipolar, multicentric terms, up to quadrupole. It has been shown that the system can be represented by a relatively small number of point charges, one for each atom and one for each chemical bond [25]. The electrostatic term is the sum of all intermolecular multipole-multipole interactions. 

Charge density analyses can provide experimental information on the concentration of electron density around atoms and in intra- and intermolecular bonds, including the location of lone pairs. Transition metal d-orbital populations can be estimated from the asphericity of the charge distribution around such metal centers. A number of physical properties that depend upon the electron density distribution can also be calculated. These include atomic charges, dipole and higher moments, electric field gradients, electrostatic potentials and interaction... 

In conclusion, it is clear that considerable information about a molecule s inherit stability, and its ability to interact with other chemical species, can be deduced from the electrostatic potential and some other well defined properties that reflect the molecular charge distribution. It should be emphasized that this approach only requires the wavefunction of the isolated molecule to be calculated, and it is therefore considerably more economical than the conventional supermolecule approach for calculation of intermolecular interaction energies. In particular, we believe that this methodology can be very useful for studying interactions in biological systems, since these often involve large molecules with several interaction sites. 

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. 

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