I'am using QE for surface study, however to preserve calculation timing and RAM usage, I need to fixed/freeze the atoms constituted by the central layer. For example, I cut FCC Co2FeSi along the lattice direction 001 with 13 layers and 15 angstrom as a vacuum. I am confused or don't know to freeze the central 3 layers using QE.

I will be grateful if anyone can help me in this query.

Thanks in Advance.