How to get core and shell coordiantes in the relax fit with core shell model?

Dear All,
I am going to fit the Buckingham potential paramters of ZrSiO4, mimicing the official example 14 of GULP.
However, taking the example 14 as an example, how do I get the core and shell coordinates of any atom, just looking as:

Si core 0.468200 0.000000 0.333333 4.000000
O core 0.413100 0.266100 0.213100 0.869020
O shel 0.431813 0.264902 0.204106 -2.869020
(The last value in each line is charge.)

Thanks for your kind help and precious time.
Shuai Nan.

Hi Shuai Nan,
The first thing to say is that there are quite a few models out there fore zircon and so you need to be clear about what you are planning to do that is better than the previous models?
The coordinates for atoms (cores) comes from the crystal structure. As for the shells, you need to read the Phil. Mag. B reference for GULP that describes fitting since this topic is covered in this article along with the different methods for handling shells.

Hi, Julian,
Thanks for your reply.
As you say, there are several papers deseribing the Buck popential parameters of zircon. However, when I used them to simulate the melt process, it was problematic. So I want to fit its parameters.
I will read the references you mentioned carefully.
Thank you again.