Question about calculating deformation potential constant

Dear developers:

I am a little bit confused about how to get volume deformation potential. I have read your paper, it says:
“To account for shifts in the average electrostatic potential between deformed cells, we align the eigenvalues to the energy level of the deepest core state.”
I also checked the paper you cited (ref. 58), it says:
“we calculate the deformation potential of the transition between the VBM and the averaged cation and anion 1s core levels”.
Still, I don’t understand quite well about these words. Could you explain more about how to align the eigenvalues to the energy level of the deepest core state? What should I calculate?
By the way, I noticed the volume deformation potential given by you and ref. 58 at VBM and CBM have obvious difference, Could you explain it?

Best regards,
Tao