ABINIT Discussion Forums

Hi,
I am a new to abinit, and during the learning the tutorials I have some problems in the tutorial #4, and they had troubled me for a long time.
I deeply hope someone can answer me .

1. About the parameter rprim sets:
In t43.in file ,the parameter rprim is seting
rprim 0.0 0.5 0.5
0.5 0.0 0.5
0.5 0.5 0.0
,however , in t44.in file ,it sets
rprim 0.5 -0.5 0.0
0.5 0.5 0.0
0.0 0.0 1.0
and in t45.in , it sets the last line 0.0 0.0 2.0, Can you tell me why ?

2. About the parameter shiftk sets:
In t43 , the shift is seting :
nshiftk 4
shiftk 0.5 0.5 0.5
0.5 0.0 0.0
0.0 0.5 0.0
0.0 0.0 0.5
but in t44, it sets
nshiftk 2
shiftk 0.5 0.0 0.5
0.0 0.5 0.5
and in t45, it becomes
nshiftk 2
shiftk 0.5 0.0 0.0
0.0 0.5 0.0

Can you tell me what I should base on to set it?

3. About the supercell:
I do not understand the defiitione of the supercell very well , and in the literature there is 2*2*2 supercell ,and what does it mean?
During t44.in file , it says it have set up a supercell, and can you tell me which parameter is for the supercell setting ？In other words ,through which parameter can we find the supercell setting?

I know these questions which asking are stupid, but I really do not understand . Also can you recommend some books or literature to me for the abinit learning ?
Thank you very much!

Hi Paul,

1. About the parameter rprim sets:
In t43.in file ,the parameter rprim is seting
rprim 0.0 0.5 0.5
0.5 0.0 0.5
0.5 0.5 0.0
,however , in t44.in file ,it sets
rprim 0.5 -0.5 0.0
0.5 0.5 0.0
0.0 0.0 1.0
and in t45.in , it sets the last line 0.0 0.0 2.0, Can you tell me why ?

The vectors
rprim
0.0 0.5 0.5
0.5 0.0 0.5
0.5 0.5 0.0
together with the atomic coordinates
xred 0.0 0.0 0.0
define the face centered cubic crystal structure of aluminum. Now, we want to introduce a layer of vacuum in the z direction. This requires that we can repeat some layer of atoms with a periodicity larger than the thickness of the layer. To do this easily, we would like to have a set of vectors defining our crystal structure among which one is in the z direction. The crystal structure defined by
rprim
0.5 -0.5 0.0
0.5 0.5 0.0
0.0 0.0 1.0
xred
0.0 0.0 0.0
0.5 0.5 0.5
does just that : the crystalline structure generated is exactly the same as the preceding one, but we have one vector in the z direction. Increasing it's size to (0.0 0.0 2.0) will cause the layer of atoms defined by xred and the first 2 rprim vectors to be repeated with a layer of vacuum between each.

2. About the parameter shiftk sets:
In t43 , the shift is seting :
nshiftk 4
shiftk 0.5 0.5 0.5
0.5 0.0 0.0
0.0 0.5 0.0
0.0 0.0 0.5
but in t44, it sets
nshiftk 2
shiftk 0.5 0.0 0.5
0.0 0.5 0.5
and in t45, it becomes
nshiftk 2
shiftk 0.5 0.0 0.0
0.0 0.5 0.0

Can you tell me what I should base on to set it?

These values of shiftk define Monkhorst-Pack grids, as mentioned in the help on the keyword :
http://www.abinit.org/documentation/helpfiles/for-v6.8/input_variables/varbas.html#shiftk
They are explained in the article:
http://link.aps.org/doi/10.1103/PhysRevB.13.5188
The idea is to carry out integrations on the Brillouin's zone more efficiently than with a simple homogeneous sampling. However, if the speed of your calculations are not a major issue to you, you can just use :
ngkpt 8 8 8
nshiftk 1
shiftk 0.0 0.0 0.0
instead of :
ngkpt 4 4 4
nshiftk 4
shiftk 0.5 0.5 0.5
0.5 0.0 0.0
0.0 0.5 0.0
0.0 0.0 0.5
to obtain results at least as good, but with more k-points.

3. About the supercell:
I do not understand the defiitione of the supercell very well , and in the literature there is 2*2*2 supercell ,and what does it mean?
During t44.in file , it says it have set up a supercell, and can you tell me which parameter is for the supercell setting ？In other words ,through which parameter can we find the supercell setting?

A supercell is a unit cell bigger than the primitive cell. For instance, the unit cell defined in t44.in is a supercell of the primitive cell defined in t43.in. Also, if we have a simple cubic crystal :
rprim
1 0 0
0 1 0
0 0 1
natom 1
typat 1
xred
0 0 0
We can define a 2*2*1 supercell by writing
rprim
2 0 0
0 2 0
0 0 1
natom 4
typat 1 1 1 1
xred
0 0 0
1 0 0
0 1 0
1 1 0

Dear jolafc,
Thank you very much!
Please forgive me to thanks late, because I undertand it until now~
Thanks again for your detailed answers .