Defines what calculator is used for performing geometry optimization.
By default, Octopus is used for computing the forces and energy.
However, it is possible to use (if selected at compile time) a machine-learned
universal force field to do the optimization of the geometry instead of Octopus.
Options:
internal:
Usual case: forces/stress are only computed by Octopus
mattersim:
MatterSim deep learning atomistic model.