[QE-users] crystal definition

[valeria arteaga muñiz]

Hello everyone.

I have been working for a couple of months with quantum espresso and still there are some things that get me confused. I hope you can help me.
If we wanted to define a BCC crystal (lets say) we could make use of the ibrav=3 parameter. With this, we would have a bcc structure with one atom per cell (?). I don’t know how can that be possible if the bcc cell is supposed to have two atoms per cell (1/8 contribution of  8 corner atoms + 1 contribution of a middle atom). When I use xcrysden for visualization just setting ibrav =3, I get the complete bcc structure.

I also have tried to create my bcc crystal from scratch using ibrav=0 and placing the atomic positions at the corner and other atom at 0.5 0.5 0.5 (center), which leads to 2 atoms per unit cell. When I make scf calculations for this case I get around twice the total energy I get in previous case (using ibrav=3). I wonder what is the correct way to use this.


Thanks for your clarifications,
Valeria

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