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How to select RMT radii  

2011-06-20 16:59:00|  分类: Linux |  标签: |举报 |字号 订阅

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The setting of RMT is crucial, I try to make some statement, but it is important tat you consider all points, but not one or two.

1. Choose the radii as large as possible, this will save computer time

2. Chossing 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).

3. if core charge leaks out of the spheres (check the case.scf whether too much charge is missing) you can
either:
i) 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:
ii) increase the sphere of the atom where the core charge leaks out.

4. Don't make your RMTs very different, some rules of thumb may be:
i) atoms with d(f) states may be 10-15% larger
ii) when space permits, 30% larger spheres are still OK, but when the spheres are too different (i.e. 1.3a.u and 3.0a.u), this will lead to quite different "effective" RKMAX values for these two atoms and eventually lead to approximative linear dependenes and resulting ghostbands.

5. Don't use radii larger than 3.0a.u., even when space allows.

6. Identical atoms should have the same RMTs

7. Usually the standard ionic radii are less important in determination of RMT.
 
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