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<title>GAMGI Tutorials: Random Close Packing structure</title>
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<h1>Random Close Packing structure</h1>
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<ul>
<li><span>Page 1</span></li>
</ul>
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The Random Close Packing (RCP) structure has the highest volume density among
amorphous structures (0.62 - 0.64), which compares with the highest volume
density among crystalline structures (0.74). The RCP structure is particularly
suitable to describe metallic glasses, due to the nondirectional nature
of metallic bonding combined with the absence of local charge-neutrality
requirements.
<p/>
To build a RCP structure, first we create an orthorhombic cell with
the required dimensions, then a Cu atom used as model, and then link
the cell with the atom, to create the random structure.
<p/>
The result, for 50,000 atoms, can be seen in the figure at
<a href="http://www.gamgi.org/images/screenshot12_3b.png">
http://www.gamgi.org/images/screenshot12_3b.png</a>.
<ol>
<li>
Press <b>Cell->Create</b> and set <b>System</b> to <b>o</b>,
<b>Lattice</b> to <b>P</b> (Primitive Orthorhombic), <b>a</b>
to <b>10</b>, <b>b</b> to <b>9</b> and <b>c</b> to <b>8</b>.
Press <b>Ok</b> to create the cell.
</li>
<li>
Select <b>Atom->Create</b> and set <b>Style</b> to <b>Solid</b>,
<b>Size</b> to <b>1.0</b> and <b>Variancy</b> to <b>1.0</b>.
Set <b>Element</b> to <b>Cu</b> and press the mouse over the
graphic area. A Cu atom is created where the mouse was pressed.
</li>
<li>Select <b>Light->Create</b> and press <b>Ok</b>,
to iluminate the atoms. Select <b>Layer->Modify</b>,
go to the <b>View</b> page, set <b>Scale</b> to <b>0.5</b>,
to decrease the size of the objects, and press <b>Ok</b>.</li>
<li>
Select <b>Cell->Link</b>, press the mouse over the cell
and then over the Cu atom, to identify the objects to link.
Select the <b>Random</b> link method and press <b>Ok</b>.
A RCP structure with 40 atoms is created, with density
slightly above <b>0.62</b>.</li>
<li>Press <b>Atom->Modify</b> and press the mouse sucessively
over the initial model atom and one of the atoms just created,
to compare the radius. By default, the RCP structure will contain
atoms with a radius as close as possible to the model atom
(the number of spheres is determined by GAMGI).</li>
<li>Press <b>Undo</b> (in the tools bar, below the graphic area),
press the backward arrow, and then <b>Ok</b>, to recover the
previous configuration, before the RCP structure was created.</li>
<li>Repeat the <b>Cell->Link</b> procedure, but this time
go to the <b>Packing</b> page that appear in the dialog
when the <b>Random</b> method is selected. Increase the
<b>Relaxation</b> time to <b>10,000</b> and press <b>Ok</b>.
A new RCP structure is created, with a density slightly higher
than before. In rare occasions, when the number of spheres is
very small, the density can achieve values as high as <b>0.68</b>:
the structure crystallized!</li>
<li>Increasing the <b>Relaxation</b> time gives more time
for the atoms to reorganize, improving slightly the density
but raising considerably the computation time. Repeat the whole
procedure with a larger <b>Relaxation</b> time to confirm that
the density does not keep increasing above <b>0.65</b>,
for a large number of spheres.</li>
<li>Press <b>Undo</b> again, and recover the previous configuration,
before the RCP structure was created. Repeat the <b>Cell->Link</b>
procedure, go to the <b>Packing</b> page, and press <b>Number</b>,
to fix the number of atoms to create (the radius is determined
by GAMGI). Set the number of atoms to <b>200</b>, for example,
and press <b>Ok</b>. A new RCP structure with 200 atoms is
created. As the number of spheres increased, the radius of
the spheres decreased.</li>
<li>When the number of solid spheres is large, the computing
time needed to show the structure might be too demanding. In
this case, a good tip is to select <b>Atom->Config</b> and
decrease <b>Slices</b> (the number of parallels and meridians
used to divide the sphere into plane regions), for example
to <b>4</b> (the minimum accepted).
</li>
</ol>
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