WIEN2k-FAQ: How to select RMT radii?
©2001 by P. Blaha, K. Schwarz and J. Luitz
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The determination of Rmt's is at one hand not so crucial, on the other hand
it is. Let me try to make a few statements, but it is important that you
consider all points, not just one or two.
- Choose the radii as large as possible, this will save computer time.
However:
- Choosing them smaller will make the calculations more expensive (you will
need more plane waves), but usually also more accurate (PWs are better
basis functions, reduced linearization error, better series convergence, ...).
- If core charge leaks out of the spheres (check :NEC01 in
case.scf whether too much charge is missing) you can either:
- include some high lying states as valence states by putting an
extra LO for that state (a good example is Si 2p, which is by default
in the core, but for small spheres should go into valence) or
- or increase the sphere of the atom where the core charge leaks out.
- Do not make your Rmt's too different!!! (even when geometry would allow)
Some rules of thumb may be:
- atoms with d (f) states may be 10-15% larger,
- when space permits, 30% larger spheres are still ok, but
when the spheres are too different (eg. 1.3 and 3.0 a.u.) this will lead
to quite different "effective" RKmax values for these two atoms and
eventually lead to approximative linear dependencies and resulting
ghostbands.
- For actinides,... don't use radii larger than about 3.0 a.u., even when
space allows.
- For hydrogen in very short C-H, O-H or similar bonds: Select the H sphere
radius about half of it's partner (typically C: 1.2, H: 0.6 bohr).
Reduce RKMAX (3-4 might be sufficient; check!), increase GMAX (case.in2) to 20.
- Identical atoms (elements) should have identical RMTs.
- Usually the standard ionic radii are less important in determination of Rmt.
Peter Blaha,