This variable defines which parameters are allowed to change during the optimization.
Multiple options can be chosen e.g. “ion_positions + cell_shape”.
Only one type of lattice vectors relaxation is possible.
Options:
ion_positions:
Relax position of ions
cell_shape:
Relax cell shape. This changes lattice vector lengths and directions.
cell_volume:
Relax cell volume. Only allow for rescaling the lengths of lattice vectors.