Computer simulation of cluster behaviour
A powerful tool for studying clusters is computer
simulation of their behaviour. If the nature of the
forces between the individual atoms or molecules in a
cluster is known, then one can construct a computer
model that represents the behaviour of those atoms or
molecules by solving the equations of motion of the
cluster. To describe the cluster in terms of classical
mechanics, the Newtonian equations of motion are solved
repeatedly--namely, force equals mass times
acceleration, in which the forces depend on the
instantaneous positions of all the particles. Hence,
these equations are simultaneous, interlinked equations;
there is one set of three (for the three instantaneous
coordinates of each particle) for each atom or molecule.
The results can take one of three forms: (1) the
positions and coordinates of the atoms, given in tables,
(2) the average properties of the entire cluster, or (3)
animations. Tables are too cumbersome for most purposes,
and specific average properties are frequently what the
investigator seeks. Animated sequences show the same
content as the tables but far more efficiently than
extensive tables do. In fact, animations sometimes
reveal considerably more than is expected by scientists.
It is also possible to construct computer
models of clusters based on quantum mechanics instead of
Newton's classical mechanics. This is especially
appropriate for clusters of hydrogen and helium, because
the small masses of their constituent atoms make them
very quantumlike in the sense that they reveal the
wavelike character that all matter exhibits according to
quantum mechanics. The same kinds of data and inferences
can be extracted from quantum mechanical calculations as
from classical ones, but the preparation and
visualization of animations for such clusters are much
more demanding than their classical mechanical
counterparts.
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