Structure Optimization
From EVOCD
What is structural optimization in Quantum ESPRESSO?
The idea of structural optimization is to find the structure with the minimum energy (zero force). There are two different types of structural optimization in Quantum ESPRESSO: relax and vc-relax.
The vc-relax let the cell fully relax which is both the cell parameters and atom positions to its minimum energy structure (groud state structure).
However, the relax mod will fix the cell parameters and only change the atom positions to its minimum energy structure (groud state structure).