Lennard-Jones solute-solvent systems

The obvious question to ask, then, is what are the relative time scales of the local density fluctuations and the decay of the force correlation function in compressible SCFs, and can this medium support inhomogeneous lifetime behavior To answer this question, we examined vibrational relaxation in the simple 2-dimensional Lennard-Jones solute-solvent system at the compressible state point, T = 0.55, p = 0.30 in greater detail. 

A key to both methods is the force field that is used,65 or more precisely, the inter- and possibly intramolecular potentials, from which can be obtained the forces acting upon the particles and the total energy of the system. An elementary level is to take only solute-solvent intermolecular interactions into account. These are typically viewed as being electrostatic and dispersion/exchange-repulsion (sometimes denoted van der Waals) they are represented by Coulombic and (frequently) Lennard-Jones expressions ... 

The system studied consists of one solute molecule that is tagged and N solvent molecules, each of mass m. Since we are interested in a spherically symmetrical potential, the pair potential of the solvent-solvent pair and the solute-solvent pair is assumed to be given by the simple Lennard-Jones 12-6 potential... 

The picture of a solute molecule stabilized in solution by a local environment where the solvent s concentration differs considerably from the bulk value is consistent with experiments and simulation. The encouraging agreement between the basic trends found in experiments and simulations should not obscure the fact that Lennard-Jones atoms are a pedestrian representation of the actual molecules studied in the fluorescence experiments. Caution must therefore be exercised when comparing simulations and experiments. At the same time, the very fact that such a crude model is able to capture the essential physics of the phenomenon under investigation lends further support to the notion that local density augmentations are common to all attractive supercritical systems. 

Next, intermolecular potential functions are developed to describe the interactions between the reacting system and a solvent molecule. For aqueous solutions, the potential functions are based on numerous ab initio calculations for complexes of the substrate and a water molecule. The potentials vary with Tj and are represented in our work through Coulomb and Lennard-Jones interactions between sites normally coincident with the atoms. 

The interactions between the QM part and the MM part are treated by a combination of quantum-chemical and force-field contributions. If the system can be separated into a QM solute and an MM solvent, the partitioning of interactions arises naturally The electrons of the QM part feel the electric field of the partial charges of the solvent molecules which enter the one-electron Hamiltonian as additional point charges. The dispersion attraction and the short-range repulsion are modelled by a Lennard-Jones 6-12 potential. If, on the other... 

The simplest simulated system is a Stockmayer fluid structureless particles characterized by dipole-dipole and Lennard-Jones interactions, moving in a box (size L) with periodic botmdary conditions. The results described below were obtained using 400 such particles and in addition a solute atom A which can become an ion of charge q embedded in this solvent. The long range nature of the electrostatic interactions is handled within the effective dielectric environment seheme. In this approach the simulated system is taken to be surrounded by a continuum dielectric environment whose dielectric constant e is to be chosen self consistently with that eomputed from the simulation. Accordingly, the electrostatic potential between any two partieles is supplemented by the image interaction associated with a spherical dielectric boundary of radius (taken equal to L/2) placed so that one of these... 

In Section 8.2 we discuss the main ideas behind the formalism and illustrate some of the features based on predictions from integral equation calculations involving simple binary mixtures modeled as Lennard-Jones systems (Section 8.2.1), to guide the development of, and provide molecular-based support to, the macroscopic modeling of high-temperature dilute aqueous-electrolyte solutions (Section 8.2.2), as well as to highlight the role played by the solvation effects on the pressure dependence of the kinetic rate constants of reactions in near-critical solvents (Section 8.2.3). 

The influence of the solvent is also crucial in crystallization from solution, the most common crystallization technique in practice, but, as discussed in Section 13.10, there are practically no instances in the literature where this problem has appropriately been dealt with using dynamic simulation on realistic systems - not just the Lennard-Jones fluid. It is probably appropriate to say that if real progress in predicting the appearance of crystal polymorphs is desired, the current effort in improving intermolecular potentials and search algorithms for comparing static, final crystal structures should be redirected to the study of pre-crystallization states, and to the improvement of dynamic methods for the simulation of the kinetics of crystal nucleation and growth. 

---