Lennard-Iones potential dynamics

Recently, detailed molecular pictures of the interfacial structure on the time and distance scales of the ion-crossing event, as well as of ion transfer dynamics, have been provided by Benjamin s molecular dynamics computer simulations [71, 75, 128, 136]. The system studied [71, 75, 136] included 343 water molecules and 108 1,2-dichloroethane molecules, which were separately equilibrated in two liquid slabs, and then brought into contact to form a box about 4 nm long and of cross-section 2.17 nmx2.17 nm. In a previous study [128], the dynamics of ion transfer were studied in a system including 256 polar and 256 nonpolar diatomic molecules. Solvent-solvent and ion-solvent interactions were described with standard potential functions, comprising coulombic and Lennard-Jones 6-12 pairwise potentials for electrostatic and nonbonded interactions, respectively. While in the first study [128] the intramolecular bond vibration of both polar and nonpolar solvent molecules was modeled as a harmonic oscillator, the next studies [71,75,136] used a more advanced model [137] for water and a four-atom model, with a united atom for each of two... 

A model of immiscible Lennard-Jones atomic solvents has been used to study the adsorption of a diatomic solute [71]. Subsequently, studies of solute transfer have been performed for atoms interacting through Lennard-Jones potentials [69] and an ion crossing an interface between a polar and a nonpolar liquid [72]. In both cases the potential of mean force experienced by the solute was computed the results of the simulation were compared with the result from the transition state theory (TST) in the first case, and with the result from a diffusion equation in the second case. The latter comparison has led to the conclusion that the rate calculated from the molecular dynamics trajectories agreed with the rate calculated using the diffusion equation, provided the mean-force potential and the diffusion coefficient were obtained from the microscopic model. 

This is not a finished story, bnt rather a preliminary report of work in progress. The problem of describing ion-specific effects within the framework of explicit-solvent, atomistic and classical molecnlar dynamics simulations is an ongoing endeavour that keeps evolving in close dialogue with new experimental results and systems. At the end of that journey, with faster computers, maybe optimal classical force fields will have emerged that closely reproduce all experimental ion-specific effects Maybe one soon hits a limit where it must be realised that ion-specific effects cannot be cast into a classical description based on pair-wise Lennard-Jones potentials between all constiment atoms Probably a mix of both scenarios will be realised. 

One of the first studies of multiple ions at the water/solid interface was by Spohr and Heinzinger, who carried out a simulation of a system of 8 Li" and 81" ions dissolved in 200 water molecules between uncharged flat Lennard-Jones walls.However, the issues discussed in their paper involved water structure and dynamics and the single-ion properties mentioned earlier. No attempt was made to consider the ions distributions and ion-ion correlations. This work has recently been repeated using more realistic water-metal potentials. ... 

Rafizadeh et al [68] have examined the lattice dynamics of jS-NaNa in terms of the rigid-ion (or atomic) and rigid-molecule models (see Appendix). Both approaches included coulomb forces explicitly and, where possible, used analytical potential functions of the Lennard-Jones type, (r) = 4e[(a/r) - (a/r) ], where e is the well depth and o is the distance of closest approach. The parameters for Na-Na and N-N (intermolecular) interactions were fixed from lattice-dynamic results for pure sodium and a-N2. The N3-N3 interactions in the rigid-... 

A good model of the electrolyte must describe the ions, solvent molecules and their orientation at the molecular level. Molecular dynamics simulations that are performed to visualize the orientation of the electrolyte molecules in the vicinity of the electrode surface are based on a set of parameters that can be varied in order to best described the properties of the system under investigation. The most reasonable models for solvent-solvent and ion-solvent interactions consider distribution of point charges on solvent molecules and take into account Lennard-Jones-type potentials that are strongly repulsive at short distances. Molecular dynamics simulations are typically performed on a system confined between two metal electrodes and the number of confined ions and solvent molecules is often limited by the computing power of modem computers. Some representative examples of results of such calculations are given in ref 60,63-68. 

Chandra (Chandra, 2000) investigated the specific role of ions on H-bonds between water molecules using molecular dynamics. The systems chosen were NaCl and KCl in water at various concentrations (from OM to 3.35M). Water molecules were modeled by the extended simple point charge (SPC/E) potential and ions were modeled as charged Lennard-Jones particles. For analyzing the hydrogen bond breaking dynamics, the author calculated the time correlation functions Shb(0 and S HB(f). Shb(0 describes the probability that an initially... 

The fnonbonded scrics of terms proportional to R, R and where R is the interatomic distance, with some coefficients depending on effective charges and fitted to ab initio results. For the water-solute interaction energy the Lennard-Jones (6-12) and Coulombic ab initio potentials are used, and for water-water the MCY potential is applied. This force field has been tested on molecular dynamics and minimization of the spermine complexes with d(CGCGC)2 in the B- and Z-DNA forms in vacuo and in aqueous solution. It is not clear whether the counter-ions have been used in those calculations, although the set of parameters for monovalent ions is available in this parametrization. The results for the total energies of the systems showed that the interaction of... 

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