Point-charge contributions

For iron compounds, (yzz)iat and iat are amplified by approximately (1 —yc ) 10 as compared to the point charge contributions 14z and rj obtained from (4.42a) and (4.42b). Nevertheless, the lattice contribution is usually least significant for most iron compounds because it is superseded by a strong EFG from the valence electrons details will be found in Chap. 5. 

The two-center point-charge contribution, is also intuitively appealing. 

For the point-charge contributions, the accelerated convergence expression, Eq. (9.21), is used with the substitution, Eq. (9.22). The explicit expression for the point-charge contribution is then (Bertaut 1952)... 

There have been several studies for the determination of the water/solvent structure around biomolecules using the Monte Carlo method [77-84]. The band structures of several homopolypeptides have been calculated using the effective field of water molecules. In their calculations, Clementi et al. (1977) [77] represented each water molecule by 23 point charges contributing to the... 

It also results from these charges signs that the pure point charges contributions,, to the classical electrostatic interaction is always stabilizing, contrarily to the total classical electrostatic part. When going from F...N = 3.68 to F...N = 3.05 A,... 

The contribution of 0th rank is called the point charge contribution. It was the only one included in the first version of the model which was, for this reason, designated as the point charge electrostatic model (PCEM). This contribution is written as ... 

The interaction between two charges qi and qj separated by the distance rij in a medium with a dielectric constant e is given by Coulomb s law, which sums the energetic contributions over all pairs ij of point charges within a molecule (Eq. (25)). 

N is the number of point charges within the molecule and Sq is the dielectric permittivity of the vacuum. This form is used especially in force fields like AMBER and CHARMM for proteins. As already mentioned, Coulombic 1,4-non-bonded interactions interfere with 1,4-torsional potentials and are therefore scaled (e.g., by 1 1.2 in AMBER). Please be aware that Coulombic interactions, unlike the bonded contributions to the PEF presented above, are not limited to a single molecule. If the system under consideration contains more than one molecule (like a peptide in a box of water), non-bonded interactions have to be calculated between the molecules, too. This principle also holds for the non-bonded van der Waals interactions, which are discussed in Section 7.2.3.6. 

In contrast to the point charge model, which needs atom-centered charges from an external source (because of the geometry dependence of the charge distribution they cannot be parameterized and are often pre-calculated by quantum mechanics), the relatively few different bond dipoles are parameterized. An elegant way to calculate charges is by the use of so-called bond increments (Eq. (26)), which are defined as the charge contribution of each atom j bound to atom i. 

The electrostatic potential at a point r, 0(r), is defined as the work done to bring unit positive charge from infinity to the point. The electrostatic interaction energy between a point charge q located at r and the molecule equals The electrostatic potential has contributions from both the nuclei and from the electrons, unlike the electron density, which only reflects the electronic distribution. The electrostatic potential due to the M nuclei is ... 

The PCM algorithm is as follows. First, the cavity siuface is determined from the van der Waals radii of the atoms. That fraction of each atom s van der Waals sphere which contributes to the cavity is then divided into a nmnber of small surface elements of calculable surface area. The simplest way to to this is to define a local polar coordinate frame at tlie centre of each atom s van der Waals sphere and to use fixed increments of AO and A(p to give rectangular surface elements (Figure 11.22). The surface can also be divided using tessellation methods [Paschual-Ahuir d al. 1987]. An initial value of the point charge for each surface element is then calculated from the electric field gradient due to the solute alone ... 

The electric field can be incorporated in the Flamiltonian via a finite field term or approximated by a set of point charges. This allows the computation of corrections to the dipole only, which is generally the most significant contribution. 

The electron density distributions are approximated by a series of point charges. There are four possible types of contributions, i.e. 

The exact expression for the dipole moment does n( consider atoms as point charges, but rather as nuclei (eat with a positive charge equal to the atomic number) ar electrons (each with unit negative charge). Atoms wii lone pairs may contribute to the dipole moment, even the atom is neutral, as long as the lone pair electrons a not symmetrically placed around the nucleus. 

From Table 2.5 it is clearly seen that becomes small (less than 0.001 kcal/ mol) beyond a distance of 10 A. The electrostatic interaction reaches the same level of importance at a distance of 30 A. The Table also shows that the interaction between point charges behaves much like a dipole-dipole interaction, i.e. an R dependence. However, the interaction between net charges is very long range even at 100 A separation, there is a 0.34kcal/mol energy contribution. The cut-off distance corresponding to a contribution of 0.001 kcal/mol is of the order of 3000 A ... 

The EFG parameters Vzz and described by (4.42a) and (4.42b) do not represent the actual EFG felt by the Mossbauer nucleus. Instead, the electron shell of the Mossbauer atom will be distorted by electrostatic interaction with the noncubic distribution of the external charges, such that the EFG becomes amplified. This phenomenon has been treated by Stemheimer [54—58], who introduced an anti-shielding factor (1 —y 00) for computation of the so-called lattice contribution to the EFG, which arises from (point) charges located on the atoms surrounding the Mossbauer atom in a crystal lattice (or a molecule). In this approach,the actual lattice contribution is given by... 

The QM/MM interactions (Eqm/mm) are taken to include bonded and non-bonded interactions. For the non-bonded interactions, the subsystems interact with each other through Lennard-Jones and point charge interaction potentials. When the electronic structure is determined for the QM subsystem, the charges in the MM subsystem are included as a collection of fixed point charges in an effective Hamiltonian, which describes the QM subsystem. That is, in the calculation of the QM subsystem we determine the contributions from the QM subsystem (Eqm) and the electrostatic contributions from the interaction between the QM and MM subsystems as explained by Zhang et al. [13],... 

Table 2.1 Non-forbidden crystal field parameters that a group of point charges may contribute collectively, as a function of their point-group symmetry.

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