ABINIT Discussion Forums

Dear all,

I am studying bulk materials, a FCC and a BCC one. For both, I considered
a conventional unit cell
rprim
1 0 0
0 1 0
0 0 1

For the FCC, I have 4 atoms with
xred
0.0 0.0 0.0
0.5 0.5 0.0
0.0 0.5 0.5
0.5 0.0 0.5

and for the BCC I have 2 atoms with
xred
0.0 0.0 0.0
0.5 0.5 0.5

I want to change the orientation of my unit cell. In fact, I would like to consider
the (100), (110) and (111) surface orientations. The idea is to make a bulk uniaxial
relaxation along these three directions, and so to increase the distance between
theses atomic planes.

For each surface orientation, how should I change rprim and xred, to allow a
relaxation of the bulk in a direction perpendicular to the surface ?

Best regards.

Dear Hola,
There are some crystallographic programs that you can use to build your surfaces, you can handle that with VESTA for example.
Best wishes,
Eric

Dear Eric,

Thank you for your response. In the Abinit tutorial n°4, to determine the (100) surface
energy of the FCC Aluminium, they first change the orientation of the cell by defining a supercell with new primitive vectors :

rprim
0.5 -0.5 0.0 # These values will define a cell with two atoms,
0.5 0.5 0.0 # non-primitive, with a different orientation
0.0 0.0 1.0 # than the primitive cell.

with xred
0.0 0.0 0.0
0.5 0.5 0.5

and then they increase the length of the third vector :
rprim
0.5 -0.5 0.0
0.5 0.5 0.0
0.0 0.0 2.0

I am not interested by surface energy, but by bulk uniaxial relaxation. I was wondering, If I define the FCC Aluminium with conventional vectors :
acell 3*7.5
rprim
1 0 0
0 1 0
0 0 1
xred
0.0 0.0 0.0
0.5 0.5 0.0
0.0 0.5 0.5
0.5 0.0 0.5

and if I increase the lenght of the third vector with acell becoming :
acell 2*7.5 9

in this way, do I make a relaxation of the bulk along the (001) direction ?

Best regards.