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Re^2: How to improve convergence

Vyacheslav
kreme_vg@chemy.kolasc.net.ru

   Dear Sanya,
thank you very much for your detail comments. The systems similar mentioned above are necessary to me for some modelling purposes.
  I know that unsaturated bonds of a clusters are usually closed by hydrogen. In this connection I would like to ask:  when you say that coronene etc. may serves as a suitable model of graphite, what properties (besides convergence) you mean?
  One question of the more general interest to you (and to all): in connection with PAW what you think about package Quantum ESPRESSO? Perhaps, someone used it?
Thanks!

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On Sun Dec 20 '09 2:26am, sanya wrote
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>Are you using HF or DFT? Anyway, your system can hardly be treated by any of these methods. Are you really interested in an open-shell high-spin system? My experience shows that such open-shell systems should be capped by hydrogens to avoid problems with degenerate states that cause poor convergence (believe me, graphite was the subject of my M.Sc. thesis 16 years ago). Real graphite is not paramagnetic, so high-spin models are unappropriate for it. Instead, hydrocarbons with fused rings (such as coronene, etc.) can serve as very good models. They are closed-shell, and you'll have no problems with convergence. However, if you are going to use DFT for geometry optimizations, take dispersion-corrected functionals (implemented in GAMESS-US), because Van der Waals interactions (which are underestimated by conventional DFT) are very important part of plane-plane interaction in graphite.

>On the other hand, if you are still going to deal with such unrealistic open-shell models, you can start with the orbitals from the converged run (with mult=5) (GUESS=MOREAD). It may help as likely as not. Changing the value of COORD or INTTYP options has no effect on the SCF convergence. And the reliability of the results obtained for an inappropriate model is questionable.

>Alternatively, you may use some plane-wave packages (such as GPAW).

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