Different of POSCAR from API and MP website!

Dear friends,
When I query the stable elements by API by the below code:

from mp_api.client import MPRester
with MPRester(“API_Key”) as mpr:
docs = mpr.summary.search(num_elements=(1,1),chemsys=“Si”,
mpid_formula_dict = {doc.material_id for doc in docs}
for item in mpid_formula_dict:
structure = mpr.get_structure_by_material_id(item)
structure.to( filename=‘POSCAR’)
I get the below poscar file:
3.3335729999999999 0.0000000000000000 1.9246390000000000
1.1111910000000000 3.1429239999999998 1.9246390000000000
0.0000000000000000 0.0000000000000000 3.8492780000000000
0.8750000000000000 0.8750000000000000 0.8750000000000000 Si
0.1250000000000000 0.1250000000000000 0.1250000000000000 Si
But when using the MP website for stable Si structure that it’s Ehull equal to zero but the stable one is the below structure:
5.4437023729394527 0.0000000000000000 0.0000000000000003
0.0000000000000009 5.4437023729394527 0.0000000000000003
0.0000000000000000 0.0000000000000000 5.4437023729394527
0.7500000000000000 0.7500000000000000 0.2500000000000000 Si
0.0000000000000000 0.5000000000000000 0.5000000000000000 Si
0.7500000000000000 0.2500000000000000 0.7500000000000000 Si
0.0000000000000000 0.0000000000000000 0.0000000000000000 Si
0.2500000000000000 0.7500000000000000 0.7500000000000000 Si
0.5000000000000000 0.5000000000000000 0.0000000000000000 Si
0.2500000000000000 0.2500000000000000 0.2500000000000000 Si
0.5000000000000000 0.0000000000000000 0.5000000000000000 Si
As you saw the only stable (Ehull=0) is above the structure of Si. I do not know how this difference takes place and which one is the main structure that you use as the base structure for the calculation of total energy for formation energy.
Of course, a similar question asks “Structure from API and that from Website differ” but I can not sure about the correct structure and I need to know which one is the best one for calculating the formation energy of materials with Si components.

Thank you in advance for your guidance.

Hi @ehsanab
Thanks for your question! Indeed the two structures look different at first glance. However, they are the same structure. It’s just that the first one is a primitive cell and the second one is a conventional cell. If you go to the Si with 0 E_hull on the webpage (mp-149: Si (Cubic, Fd-3m, 227)), and in the “Crystal Structure” section, toggle between primitive and conventional cell, you will see both information.
API by default returns the primitive cell, which is why they seem different.
Hope this helps!

thank you for your guidance.