# Random Close Packing structure

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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.

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.

The result, for 50,000 atoms, can be seen in the figure at http://www.gamgi.org/images/screenshot12_3b.png.

1. Press Cell->Create and set System to o, Lattice to P (Primitive Orthorhombic), a to 10, b to 9 and c to 8. Press Ok to create the cell.
2. Select Atom->Create and set Style to Solid, Size to 1.0 and Variancy to 1.0. Set Element to Cu and press the mouse over the graphic area. A Cu atom is created where the mouse was pressed.
3. Select Light->Create and press Ok, to iluminate the atoms. Select Layer->Modify, go to the View page, set Scale to 0.5, to decrease the size of the objects, and press Ok.
4. Select Cell->Link, press the mouse over the cell and then over the Cu atom, to identify the objects to link. Select the Random link method and press Ok. A RCP structure with 40 atoms is created, with density slightly above 0.62.
5. Press Atom->Modify 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).
6. Press Undo (in the tools bar, below the graphic area), press the backward arrow, and then Ok, to recover the previous configuration, before the RCP structure was created.
7. Repeat the Cell->Link procedure, but this time go to the Packing page that appear in the dialog when the Random method is selected. Increase the Relaxation time to 10,000 and press Ok. 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 0.68: the structure crystallized!
8. Increasing the Relaxation 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 Relaxation time to confirm that the density does not keep increasing above 0.65, for a large number of spheres.
9. Press Undo again, and recover the previous configuration, before the RCP structure was created. Repeat the Cell->Link procedure, go to the Packing page, and press Number, to fix the number of atoms to create (the radius is determined by GAMGI). Set the number of atoms to 200, for example, and press Ok. A new RCP structure with 200 atoms is created. As the number of spheres increased, the radius of the spheres decreased.
10. 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 Atom->Config and decrease Slices (the number of parallels and meridians used to divide the sphere into plane regions), for example to 4 (the minimum accepted).

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