Lennard-Jones potential, water molecule

Very similar to the properties of the free surface are the properties of water near smooth walls, which interact only weakly with water molecules. Many different models have been used, such as hard walls [81-83], exponentially repulsive walls [84-86], and Lennard-Jones potentials of various powers [81,87-96]. 

We have studied, by MD, pure water [22] and electrolyte solutions [23] in cylindrical model pores with pore diameters ranging from 0.8 to more than 4nm. In the nonpolar model pores the surface is a smooth cylinder, which interacts only weakly with water molecules and ions by a Lennard-Jones potential the polar pore surface contains additional point charges, which model the polar groups in functionalized polymer membranes. 

Surface Potentials. Consider the form of the surface-water Interaction potential for an interfacial system with a hydrophobic surface. The oxygen atom of any water molecule is acted upon by an explicitly uncharged surface directly below it via the Lennard-Jones potential (U j) ... 

The above forms for the Lennard-Jones surface-water interaction potential have been used as models of hydrophobic surfaces such as pyrophyl1ite, graphite, or paraffin. If the intention of the study, however, is to understand interfacial processes at mineral surfaces representative of smectites or mica, explicit electrostatic interactions betweeen water molecules and localized charges at the surface become important. 

The multidimensional potential energy surface was written as the sum of a gas-phase (LEPS) energy surface incorporating the main features of the one-dimensional double-well potential in Example 10.1, solvent-solute interactions described by Lennard-Jones potentials with added (Coulomb) interactions corresponding to point charges, and solvent solvent interactions including intermolecular degrees of freedom. The solvent consisted of 64 water molecules. 

One other aspect of nonprimitive electric double layer theories which is particularly relevant to the inner Stern region are the models for the water molecule and the ions. The simplest models for a water molecule and an ion are a hard-sphere point dipole and point charge, respectively. A more realistic model of the hard-sphere water molecule would include quadrupoles and octupoles and also polarizability. However the hard-sphere property is best avoided and replaced, for example, by a Lennard-Jones potential. An alternative to a multipolar water model are three point charge sites associated with the atoms within the water molecule. 

We adopted a microcanonical ensemble MD method. The Lennard-Jones potential was used for the molecular interaction in the argon, OPLS potentials for methanol and water. We make a liquid slab with thickness of about 10 molecules at the center of the rectangular unit cell with periodic boundary conditions for all three dimentions. Figure 1 is a snapshot of a typical molecular configuration. Both sides of the slab are free liquid surfaces, on which molecules can evaporate and condense. The cell size along the surface normal is typically 100 A. and the surface area is -50 A x.50 A. The number of molecules is 1200 for argon, 864 for methanol, and 1024 for water. Other technical details are described elsewhere. ... 

In the case of ABA/AOBA acids, we have neglected the repulsive interaction between acid molecules due to their large size. In the case of the interaction of water molecules, we can keep the repulsion at least at the first stage of consideration. But what kind of potentials should we choose in the case of water Since the hydrogen bond in many aspects is similar to the ionic interaction, we may assume that the pair potential between two water molecules combines both van der Waals type of interaction and the electrostatic energy. Let us choose the potential as the sum of the Lennard-Jones potential and the ionic crystal potential ... 

The discussion in this chapter has largely concerned very simple liquids such as a hypothetical fluid composed of non-interacting hard spheres, or spheres interacting via the Lennard-Jones potential function. The most common liquid, namely, water, is much more complex. First, it is a molecule with three atoms, and has a... 

Fig. 7. Oxygen density profile from a 50 ps simulation of 385 water molecules that are confined on the left side by a simple (9-3) Lennard-Jones potential [137] and on the right side by 4 layers of mercury with a (111) surface structure.

Unfortunately, it has turned out to be exceedingly difficult to accommodate all the unique features of water within any given, classical model. This is reflected in the absence of satisfactory agreement between experiments and any given model [2]. This is a bit unusual (and of course fmstrating) because when one usually models a given molecule, such as methane, it is adequate to use a simple functional form such as a Lennard-Jones potential that incorporates a measure of size and a measure of interaction energy at an optimal separation between two molecules. In the case of water molecules, such a simple procedure does not work. Here we have to account... 

There are three different types of interactions in a solution of a non-polar solute (we label a spherical non-polar solute as A) and water (labeled as W) - namely, the A-A, A-W, and W-W interactions. In their semi-empirical approach Pratt and Chandler did not make any approximation for the water-water (W-W) interaction. As we discuss here, the required pair correlation function among water molecules is obtained from experiments using the oxygen-oxygen correlation function of pure water. However, they have considered a Lennard-Jones potential for the A-A interaction. 

Fig. 6.S. Rotational excitation may lead to creating the resonance states. As an ilhjstiation a potential eneigy curve Vkj R) of eq. (6.24) has been chosen that resembles what we would have for two water molecules bound by the hydrogen bond. Its first component Ui (R) is taken in the form of the so called Lennard-Jones potential (ct p. 287) U -(R) = with the parametets for the...

More macroscopic phenoipena of biomembranes are trackable by simplification with abandonment of atomic details in the calculations. In a simulation at oil-water interfaces, water and oil molecules are represented by particles labeled w and o , respectively. An amphiphilic molecule such as a lipid is represented by a chain of two w particles followed by five o particles. A completely repulsive interaction is assumed between o and w, and a Lennard-Jones potential is assumed between particles of the same kind. Thus, this simulation is even simpler than the pioneering simulations of atomic chain models described in Section 2.1. Thus, more macroscopic phenomena can be studied easily. Starting from a i patially random distribution of water, oil, and the amphiphilic molecules, the monolayers and micelles composed of the amphiphilic molecules were spontaneously made at oil-water interfaces as shown in Figure 3. Depletion layers between monolayers and micelles were observed, suggesting the repulsion between biomembranes by the solvation force. 

The problem was later taken up by Pierotti who calculated the cavity term by scaled particle theory and the G-, term with a Lennard-Jones potential between the solute and all the waters taken as a uniform distribution. These were calculated for small molecule gases in several liquids including water. The results were compared with experimental Henry s law constants. An expression of Henry s law in terms of a cavity potential Gc was used ... 

---