IBRION=3 : how to choose alpha (POTIM) and mu (SMASS) ?

[lahaye]

Hi all,

I'm confused about what the VASP guide says on IBRION=3.

One has to choose alphs (POTIM) and mu (SMASS) appropriately,
so that this relaxation converges and is speedy (it can outperform
IBRION=2 by a factor of two). But how to choose those parameters?

I'm confused by the advice to run IBRION=2 first, to get an idea of
the trialstep value. However, then I already have a relaxed structure
and what's the point to relax it again with IBRION=3

Then there is also the strategy by fixing SMASS=1 and find the
maximum POTIM so that the total energy does not diverge.
Then SMASS should be decreased; but while decreasing, what
should I watch to decide that I have found the right value for SMASS ?
And yet again: in order to verify the values, I have to relax the
structure each time; by the time I find the appropriate values, I have
relaxed my structure numerous times !?!?

I'm just wonder if my view is right and whether all this is worth the effort,
instead of simple and straightforward relax a structure with IBRION=2.

Thanks,
Rob.

[admin]

please have a look at the optionic.pdf file at
http://cms.mpi.univie.ac.at/vasp-workshop,
especially at the chart on page 25
as the ionic relaxation behaviour depends on the system under investigation and the quality of the start geometry, no general rules can be set up which hold for every case. You will have to do some tests if the system does not relax straightforwardly.
To check the quality of the parameters, please have a look whether the forces and the energy decreases without starting to oscillate