Force-based optimizer
quasi-Newton algorithm (RMM-DIIS) : IBRION = 1
POTIM : the step size in the steepest descent step
NFREE : s how many ionic steps are stored in the iteration history
damped molecular dynamics : IBRION = 3
\(\ddot{\vec{x}}=-2 \alpha \vec{F}-\mu \dot{\vec{x}}\)
POTIM : controls α
SMASS : dampling factor μ
μ=2 is equivalent to a simple steepest descent algorithm (maximal damping)
μ=0 corresponds to no damping
Others
IBRION = 2 : conjugate gradient
