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 in principal not so crucial when you take care that all expansions are properly converged (RKmax, LM expansion, Lvns, GMAX). However, in connection with other defaults of the code and with respect of the required computer time, proper settings of RMTs can be VERY IMPORTANT. Let me try to make a few statements, but it is important that you consider all points, not just one or two.
Unless you are very experienced and test a specific situation carefully yourself, it is highly recommended that you use the setrmt_lapw script, and accept the RMTs as set there. Since an automated script can never be perfect, small variations of these suggestions are possible and might improve the performance a bit.
  1. In order to make the calculations as fast as possible, choose the radii as large as possible, but consider possible structural changes (lattice parameter or force-minimization), such that you can do all calculations with the same RMTs. i
  2. For cohesive energies or energy differences between 2 different phases, make sure that the RMTs are set identical (and the corresponding effective RKMAX is also the same).
  3. Choosing them smaller will make the calculations more expensive (it will leed to more plane waves, i.e. the matrices become bigger), but eventually also more accurate when you stay with the default inputs (PWs are more general basis functions, the linearization error gets smaller (unless you use HDLOs), better series convergence (unless you increase Lvns or GMAX, ...). However, if core-leakage starts due to smaller spheres, the results will deteriorate.
  4. If core charge leaks out of the spheres (check :NEC01 in case.scf whether too much charge is missing) you can either:
  5. Do not make your Rmt's too different!!! (even when geometry would allow) Some rules of thumb may be:
  6. When your RMT are larger than 2.5 (2.2 for highest precission) bohr, choose LVNS=6-8 and consider setting of HDLOs for d and in particular f-states.
  7. 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.
  8. Identical atoms (elements) should have identical RMTs.
  9. The standard ionic radii are NOT important in determination of Rmt.

    Peter Blaha,