Four-site charge model

Cracknell et al. [22] proposed a two LJ site and a four-charge model (M = 2 and = 4). The four charges lie on the molecular axis joining the centers of two nitrogen atoms and they are symmetrical with respect to the molecular center of mass. The distance between two positive charges of 0.373e... 

The five site model of Singh and Kollman allowed the site on the 2-fold axis to split into two sites above and below the molecular plane. They also considered a four site model where the oxygen charge was set to zero. This simplification of the model led to tjn = 0.537 with opposite sign for the inverted lone pair sites. The oxygen to lone-pair distance was 0.279 A, the angle 65.5°, and the rrms fit to the electric potential 7.5%. 

Since AS is different for models with and without out-of-plane charge (Fig. 9.8, open symbols, the results are plotted as /XoS k since /u-o in the liquid is not known), three- and four-site models are only able to match experiment for e at 5 0.63, while five-site... 

First, we test four different cases to study the solvent effects. In case 1 the solvent is the simple, repulsive-potential system described above. In case 2 the solvent is the L-J potential system. The solvents treated in cases 1 and 2 can be simple models of non-polar solvents. In case 3 the solvent is the SPC/E water but all the site-charges of Met-enkephalin are set at zero to shut off the electrostatic interaction between Met-enkephalin and the solvent. In case 4 the solvent is the SPC/E water and the full values are assigned to the site-charges. It is assumed that Met-enkephalin is un-ionized even in case 4, because we have found... 

Kristof et al. [246] proposed an empirical force field, fitted to experimental molecular geometry and vapor-liquid equilibrium properties. This force field consists of one U 12-6 site plus four partial charges. Recently, Zhang and Siepmann [247] proposed a five-site ammonia force field based on the geometry of the Impey and Klein [108] model. This force field also consists of one LJ 12-6 site and four partial charges, three of them located at the hydrogen positions and one located at a... 

Figure 22 Coarse-graining strategy used to reduce the number of interaction sites required to model an ionic liquid. In (a) the cation is represented with four sites A, B, C, and E, while the anion is represented with a single site D. In (b) the simplified coarsegrained model is depicted with associated site charges. (From Ref. 128 and used with permission.)...

The model of Sprik and Klein has a set of four fluctuating charges distributed on a tetrahedron located at the M site of a TIP4P water. These charges fluctuate such that the net charge is fixed. This scheme is described by an extended Lagrangian so that the equations of motion determine the fluctuations in the effective dipole moment. A thermostat was not employed in this study, so it should be viewed as a test case only. Liquid state water properties were adequately reproduced. 

Williams, D. E. J. Chem. Phys. 1967,47,4680-4684 Wdliams, D. E. Starr, T. L. Calculation of the crystal structures of hydrocarbons by molecular packing analysis, Comp. Chem. 1977,1,173-177 Hsu, L.-Y. Wdliams, D. E. Intermolecular potential-function models for crystalline perchlorohydrocarbons, Acta Cryst. 1980, A36, 277-281 Cox, S. R. Hsu, L.-Y. Williams, D. E. Nonbonded potential function models for crystalline oxohydrocarbons, Acta Cryst. 1981, A37, 293-301 Williams, D. E. Cox, S. R. Nonbonded potentials for aza-hydrocarbons the importance of coulombic interactions, Acta Cryst. 1984, B40, 404 17 Williams, D. E. Houpt, D. J. Fluorine nonbonded potentials derived from crystalline perfluorocarbons, Acta Cryst. 1986, B42,286-295. The Williams force field requires the calculation of atomic point and site charges for each molecule. The hydrocarbon force field was later improved (Williams, D. E. Improved intermolecular force field for crystalline hydrocarbons containing four- or three-coordinates carbon, J. Mol. Struct. 1999,485-486, 321-347)... 

---