Chemistry Reference
In-Depth Information
involve the solution of Schrödinger's equation for each electron, in the
self-consistent potential created by the other electrons and the nuclei. Ab
initio methods can be applied to a wide range of systems and properties.
However, these techniques are computationally exhaustive, making them
difficult for simulations involving large numbers of atoms. There are three
widely used procedures in Ab initio simulation. These procedures are sin-
gle point calculations, geometry optimization, and frequency calculation.
Single point calculations involve the determination of energy and wave
functions for a given geometry. This is often used as a preliminary step
in a more detailed simulation. Geometry calculations are used to deter-
mine energy and wave functions for an initial geometry, and subsequent
geometries with lower energy levels. A number of procedures exist for
establishing geometries at each calculation step. Frequency calculations
are used to predict Infrared and Raman intensities of a molecular system.
Ab initio simulations are restricted to small numbers of atoms because of
the intense computational resources that are required.
Ab initio techniques have been used on a limited basis for the predic-
tion of mechanical properties of polymer based nano structured compos-
ites.
9.2.1.2
MOLECULAR DYNAMIC
MD is a simulation technique that used to estimate the time depended
physical properties of a system of interacting particles (e.g., atoms, mol-
ecules, etc.) by predict the time evolution. MD simulation used to inves-
tigating the structure, dynamics, and thermodynamics of individual mol-
ecules.
There are two basic assumptions made in standard molecular dynamics
simulations.
Molecules or atoms are described as a system of interacting material
points, whose motion is described dynamically with a vector of instan-
taneous positions and velocities. The atomic interaction has a strong de-
pendence on the spatial orientation and distances between separate atoms.
This model is often referred to as the soft sphere model, where the softness
is analogous to the electron clouds of atoms.
