Structural Simulation using Pair Potentials Energy Calculation

Over the past 15 years, a large number of papers have appeared dealing with computer simulations of water structure, thanks to the increased capacity and availability of fast computers. Simulations are based on accepting a reasonable expression for the pairwise interaction of water molecules, namely, the pair potential (energy) function. Much has been learned from simulation studies, and the results are valuable hints as to what the structure of liquid water may be, but not necessarily as to what the structure must be. Because of computational limitations, the typical sample of water molecules used in such analyses is about 500. If these molecules formed a small droplet, the radius would be five molecules, half of which would be from the outside layer. Thus, one must expect some dramatic surface effects. Furthermore, as discussed below, the calculations are based on a pairwise potential energy function this is probably a most serious limitation, as also discussed below. 

The simulation of structures using pair potential methods gives important information, including unit cell dimensions, atomic positions and details of atomic motion including lattice vibrations (phonon modes). Further analysis permits the calculation of heat capacities, the dependence of volume with temperature and the prediction of vibrational spectra, such as IR and neutron spectroscopies. Codes that perform such periodic structure energy minimisation using pair potential models include METAPOCS, THBREL and GULP (Table 4.1). All have been used successfully to model framework structures. 

To be within a certain reliability level for the calculations, simulated UV-response was compared with experimental results from [8-10]. In order to know if the radicals can bind to C60 fullerene, molecular electrostatic potentials and the transition state structures were calculated by molecular dynamics methods, using a complete LST/QST algorithm for the TS search for each one of the reactant-product pairs. The energies of the reaction are shown in Table 1. 

Two sets of methods for computer simulations of molecular fluids have been developed Monte Carlo (MC) and Molecular Dynamics (MD). In both cases the simulations are performed on a relatively small number of particles (atoms, ions, and/or molecules) of the order of 100simulation supercell. The interparticle interactions are represented by pair potentials, and it is generally assumed that the total potential energy of the system can be described as a sum of these pair interactions. Very large numbers of particle configurations are generated on a computer in both methods, and, with the help of statistical mechanics, many useful thermodynamic and structural properties of the fluid (pressure, temperature, internal energy, heat capacity, radial distribution functions, etc.) can then be directly calculated from this microscopic information about instantaneous atomic positions and velocities. 

Classical ion trajectory computer simulations based on the BCA are a series of evaluations of two-body collisions. The parameters involved in each collision are tire type of atoms of the projectile and the target atom, the kinetic energy of the projectile and the impact parameter. The general procedure for implementation of such computer simulations is as follows. All of the parameters involved in tlie calculation are defined the surface structure in tenns of the types of the constituent atoms, their positions in the surface and their themial vibration amplitude the projectile in tenns of the type of ion to be used, the incident beam direction and the initial kinetic energy the detector in tenns of the position, size and detection efficiency the type of potential fiinctions for possible collision pairs. 

---