I am interested in performing optimization at constant pressure on FAU zeolite by using SLC potential. In my input file i need to specify the fractional coordinates. I use pymatgen to find the fractional coordinates from the cif files on python where I have 576 atoms of Silicon and Oxygen. How do I know the fractional coordinates for Si core, O core, and O shell out of all the 576 atoms?

The coordinates for cores are just the crystallographic coordinates from the CIF file. The shells are a mimic for electronic effects & so the usual thing is to start them at the same coordinates as the corresponding core (see example files that come with GULP).

Thank you for your response. Does that means the 576 atoms from the fractional coordinates of the cif files is what i will use and how will i specify the Si core, O core, O shell fractional coordinates because the O core and O shell coordinates should be the same

I was able to resolve this and itâ€™s explained below. Get the primitive structure by using pymatgen on python. It will show the coordinates of the Silicon to be 48 and the Oxygen to be 96. This means the silicon core atoms is 46 with its coordinates. The oxygen core atoms is 96 with its coordinates. Also the oxygen shell is 96 with the coordinates same as oxygen core. The total coordinates will be 240.