|no default, must be set by user||if IBRION=0 (MD)|
|POTIM= 0.5||if IBRION=1,2,3 (relaxation)|
In case IBRION=0 (MD) , POTIM specifies the time step in fs. For IBRION=1,2 or 3, POTIM serves as a scaling constant for the forces.
POTIM supplies the time step for an ab-initio molecular dynamics (IBRION=0), and must be entered by the user for all MD simulations.
In addition POTIM severs as a ``scaling constant'' in all minimization algorithms (quasi-Newton, conjugate gradient, and damped molecular dynamics). Especially the Quasi-Newton algorithm is sensitive to the choice of this parameter (see section IBRION 6.22).