WIEN2k-FAQ: How to select RMT radii?
©2001 by P. Blaha, K. Schwarz and J. Luitz
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.
In addition, it is highly recommended that you use the setrmt_lapw
script, which will make a proper selection for you.
- In order to save computer time, choose the radii as large as possible.
- 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 "touch .lcore" to form a proper superposition of the core densities
leaking out of the sphere during scf.
- 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 10-20% 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
ghostbands (QTL-B error).
- When your RMT are larger than 2.5 (2.2 for highest precission) bohr,
choose LVNS=6 and consider setting of HDLOs for d and in particular f-states.
- 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 is sufficient; check!), accept the automatically increased
GMAX (case.in2) or use an even bigger one.
- Identical atoms (elements) should have identical RMTs.
- The standard ionic radii are NOT important in determination of Rmt.