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 a little bit more accurate (PWs are better basis functions, reduced linearization error).
- 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.
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