# How to build amorphous Al2O3 using the AIMD method implemented in VASP?

I want to build amorphous Al$$_2$$O$$_3$$ with the AIMD (ab-initio molecular dynamics) method implemented in VASP. I know that two procedures should be taken into account, namely

• 1st: melt
• 2nd: quench

Following this workflow, I encounter some questions as follows:

How big should the melt supercell take?

The conventional cell of Al$$_2$$O$$_3$$ is taken from the materials project website, in which it contains 15 atoms, as shown in the following figure.

To do AIMD calculations, we must contain enough atoms in the simulated cell. It means that I need to build a supercell. For the current system, a $$3 \times 3 \times 1$$ (135 atoms) supercell is OK? To obtain a reasonable amorphous structure, how big the melt supercell should take?

Once the supercell is built, I need to heat the supercell to melt it.

How to take the beginning temperature (TEBEG=X1) and ending temperature (TEEND=X1)? How to set the timestep (POTIM=X2 fs) and simulation numbers (NSW=X3)?

If the previous two questions are solved, I have a melting structure. Now I need to quench the structure to obtain the amorphous structure, also by doing AIMD.

How to set the beginning temperature (TEBEG=X1) and ending temperature (TEEND=X5)? How to set the timestep (POTIM=X2 fs) and simulation numbers (NSW=X3)? How to judge the final structure?

If the workflow is wrong, please help me to correct it.

Many thanks in advance.

• +1. But I just changed the title slightly. There's a few disadvantages of using MathJax in titles, such as disqualifying the question from appearing on the Hot Network Questions list: mattermodeling.meta.stackexchange.com/a/190/5 – Nike Dattani Oct 30 '20 at 19:38
• How "amorphous" your structure is will depend on the cell size. Do you intend to make a surface out of it? I think this question is answerable as is, but some slightly more detailed goals might help. If you need a surface, it may affect things differently. – Tristan Maxson Oct 31 '20 at 4:06
• @TristanMaxson:researchgate.net/publication/… Please take a look at this paper. The amorphous Fe2O3 is built from AIMD. – Jack Oct 31 '20 at 12:54