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POTIM = [real]
|Default: POTIM||= none,||must be set if IBRION= 0 (MD)|
|= 0.5||if IBRION= 1, 2, and 3 (ionic relaxation), and 5 (up to VASP.4.6)|
|= 0.015||if IBRION=5, and 6 (as of VASP.5.1)|
Description: POTIM sets the time step in molecular dynamics or the step width in ionic relaxations.
- For IBRION = 0, POTIM gives the time step (in fs) in all ab-initio Molecular Dynamics runs, it has to be supplied therefore, otherwise VASP crashes immediately after having started.
- For IBRION =1, 2, and 3, which corresponds to ionic relaxation using a quasi-Newton algorithm, conjugate-gradient algorithm, and damped molecular dynamics, respectively, the POTIM tag serves as a scaling constant for the step widths. Especially the quasi-Newton algorithm is sensitive to the choice of this parameter.
- For IBRION = 5, and 6, a phonon calculations using the finite differences approach is done, where POTIM is the width of the displacement of each ion to calculate the Hessian Matrix.
- VASP.4.6 and older releases: POTIM has to be small enough to ensure that the displacements are within the harmonic limit. The vibrational frequencies using the frozen phonon approach are based on the harmonic approximation.
Mind: For VASP.5.1 and newer releases, POTIM is automatically reset to 0.015 Å, if the supplied value for POTIM is unreasonably large.