[Wien] O2 molecule continue (rmt issue)

[Laurence Marks]

"besides the statement: be careful with the energy parameter (very fine
E-search in case.in1)"
N.B., one does not need a fine search in the more recent Wien2k, only in
the older versions.

I seem to remember that GMax 20 mainly cancels out for LDA/GGA -- I would
always use it (or more) for a meta GGA.

_____
Professor Laurence Marks
"Research is to see what everybody else has seen, and to think what nobody
else has thought", Albert Szent-Gyorgi
www.numis.northwestern.edu

On Nov 13, 2017 2:00 PM, "Peter Blaha" <pblaha at theochem.tuwien.ac.at> wrote:

> 1- Computing the atomic energies of atoms like N and P in an FCC cell is
> ok, however for O atom the high symmetry of the FCC cell results in 1/3
> occupancies (for the 4th p electron of O) in the spin down case. Only
> using a lower symmetry cell (orthorhombic) for O atom eliminates this
issue.

This is true, however, for O this is a very small (negligible) energy
difference. It is a bit more important (also to help convergence) for
transition metals or even lanthanides.

> 2- For very small RMTs (e.g. 1.1 for O) one needs a large value of Gmax
> (~20).

Again true. GMAX for small spheres has some effect on E-tot. I have not
cross checked if this truncation cancels out in the energy differences.

> Using RMT = 1.1 for O in a compound will require one to use too small
> RMTs for large atoms. For example if we use RMT = 1.1 for O in compounds
> like SnO2, this will require one to use an RMT ~ 1.6 for Sn (RMTs
> difference should not be > 30 %). This means that the ecut should be set
> < -9.8 Ry which includes the 4s electrons to valence states. What are
> the consequences?

Without test: I would "break" my own rules and set the Sn sphere still
to 2.0 or 2.2 (such that no core leakage occurs at ecut=-6). I would not
expect ghostbands coming up. This rule usually applies to cases where
one atom could have very large RMT (2.5 or even larger) and probably
converges rapidly (eg. alkali(earth) atoms in TM perovskites, ...)

But in principle there is nothing to say against Sn 4s as valence,
besides the statement: be careful with the energy parameter (very fine
E-search in case.in1) and make sure that RKmax is sufficient for such
localized states.


>
> Best Regards,
>
> Alay
>
>
>
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--
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