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`IBRION` some general comments (`ISIF`, `POTIM`)

For `IBRION`=1,2 and 3, the flag `ISIF`
(see section 6.24)
determines whether the ions and/or the cell shape is
changed. Update of the cell shape is supported for
molecular dynamics (`IBRION`=0) only if the dynamics module of Tomas Bucko (precompiler flag `-Dtbdyn`)
is used.

Within all relaxation algorithms (`IBRION`=1,2 and 3) the parameter
`POTIM` should
be supplied in the INCAR file.
For `IBRION`0, *the forces are scaled internally before calling
the minimization routine*. Therefore for relaxations, `POTIM` has
no physical meaning and serves only as a scaling factor. For many systems,
the optimal `POTIM` is around 0.5.
Because the Quasi-Newton algorithm and the damped algorithms are sensitive
to the choice of this parameter,
use `IBRION`=2, if you are not sure how large the optimal `POTIM` is.

In this case, the OUTCAR file and stdout will contain a line
indicating a reliable `POTIM`. For `IBRION`=2, the following lines will be written to stdout
after each corrector step (usually each odd step):

trial: gam= .00000 g(F)= .152E+01 g(S)= .000E+00 ort = .000E+00
(trialstep = .82)

The quantity `gam` is the conjugation parameter to the previous step,
`g(F)` and `g(S)` are the norm of the force respectively the norm of the stress tensor.
The quantity `ort` is an indicator whether this search direction is orthogonal
to the last
search direction (for an optimal step this quantity should be much smaller
than (`g(F)` + `g(S)`).
The quantity `trialstep` is the size of the current trialstep.
This value is the average step size leading to a line minimization
in the previous ionic step. An optimal `POTIM` can be determined, by
multiplying the current `POTIM` with the quantity `trialstep`.
After at the end of a trial step, the following lines are
written to stdout:

trial-energy change: -1.153185 1.order -1.133 -1.527 -.739
step: 1.7275(harm= 2.0557) dis= .12277
next Energy= -1341.57 (dE= -.142E+01)

The quantity `trial-energy change` is the change of the energy in the trial step.
The first value after `1.order` is the expected energy change calculated from the
forces (
change of positions).
The second and third value corresponds to
change of positions
and
change of positions.
The first value in the second line is the size of the step leading
to a line minimization along the current search direction. It is calculated
from a third order interpolation formula using data
form the start and trial step
(forces and energy change). `harm` is the optimal step using
a second order (or harmonic) interpolation. Only information on the forces
is used for the harmonic interpolation.
Close to the minimum both values should be similar.
`dis` is the maximum distance moved by the ions in fractional (direct) coordinates.
`next Energy` gives an indication how large the next energy should be (i.e.
the energy at the minimum of the line minimization), `dE` is the estimated energy change.
The OUTCAR file will contain the following lines, at the end of each trial step:

trial-energy change: -1.148928 1.order -1.126 -1.518 -.735
(g-gl).g = .152E+01 g.g = .152E+01 gl.gl = .000E+00
g(Force) = .152E+01 g(Stress)= .000E+00 ortho = .000E+00
gamma = .00000
opt step = 1.72745 (harmonic = 2.05575) max dist = .12277085
next E = -1341.577507 (d E = 1.42496)

The line `trial-energy` change was already discussed.
`g(Force)` corresponds to `g(F)`, `g(Stress)` to `g(S)`, `ortho` to `ort`,
`gamma` to `gam`.
The values after gamma correspond to the second line (step: ...) previously
described.

N.B. Requests for support are to be addressed to: vasp.materialphysik@univie.ac.at