[Wien] La f-local orbital energy effects on spin polarized calculation

[omar de la peña seaman]

Dear Wien2k community,

I am running Wien version 05.6 on a Opteron Cluster with operating
system Scientific Linux 4.x with IWR and CERN patches, and PGI 7.2.2.
fortran compiler.

I'm performing a series of calculations on a rhombohedral 12-atom unit
cell LaCoO3 system (symmetry group:167 R-3c),  with a=10.351349 a.u.
c=24.742106 a.u. (in hexagonal lattice parameters, as needed in the
Wien code), and dx=0.0522 (0.25-dx, 0.25+dx, 0.75) as the input
internal parameter for the O-atom.
The purpose of my calculations is to find the optimal internal
parameter (dx) for each magnetic configuration, non-magnetic(NM) and
ferromagnetic(FM), at these specific external structural parameters.

As I said, the calculations were done for the NM (min -j "run_lapw -I
-cc 0.00001 -ec 0.00001 -fc 0.10 -i 60") and FM (min -j "runsp_lapw
-cc 0.00001 -ec 0.00001 -fc 0.10 -i 60") configurations with the
PBE-GGA xc-functional.
As documented in the Wien-mailinglist and the UG, it is recommended to
include a local orbital for the La-f electrons at quite "high energy"
(of the order of 1-3 Ry), to improve the description of unoccupied
states and reduce linearization errors.
I have tried 2 sets of calculations:
1) NM and FM with La f-local orbital at 1Ry (fixed)
 0.30    6  0      (GLOBAL E-PARAMETER WITH n OTHER CHOICES, global APW/LAPW)
 0   -2.56      0.010 CONT 1
 0    0.30      0.000 CONT 1
 1   -1.30      0.010 CONT 1
 1    0.30      0.000 CONT 1
 2    0.30      0.010 CONT 1
 3    1.00      0.000 CONT 1  <==== La f-local orbital
2) NM and FM with La f-local orbital at 3Ry (fixed)
 0.30    6  0      (GLOBAL E-PARAMETER WITH n OTHER CHOICES, global APW/LAPW)
 0   -2.56      0.010 CONT 1
 0    0.30      0.000 CONT 1
 1   -1.30      0.010 CONT 1
 1    0.30      0.000 CONT 1
 2    0.30      0.010 CONT 1
 3    3.00      0.000 CONT 1  <==== La f-local orbital
all with the same structural and numerical parameters (given at the
end of the message).

The results of the internal optimization  for each set of calculations
(energy of f-local orbital) are the following (0.25-dx):
              1 Ry                  3 Ry
NM     0.18701389       0.20843091
FM     0.19283528       0.21605310

and the energy results, which are quite interesting (units: Ry):
                 1 Ry                     3 Ry
NM     -40468.398065      -40468.173848
FM     -40468.396233      -40468.180753
Diff            -0.001832              0.006905

where the energy corresponds to the case once the optimization was
done. Diff is defined as "E(NM)-E(FM)". Then, if I put the f-local
orbital at 1 Ry, the NM is slightly more stable than the FM (~ 1.8
mRy), but if I put the f-local orbital at higher energy, then the FM
is the most stable one (by ~ 6.9 mRy).
I was quite surprised by these results, since in principle this
unoccupied state (or its position in energy) should not affect the
overall description of the system, specially on the optimization
(force optimization) of the Oxygen internal position.

So my main question is how should I realize which one is the "correct"
one? and why the position in energy of the f-local orbital affects the
total energy, in such a way that ordering or the magnetic phases are
changed? Is there any criteria or energy range for choosing the
f-local orbital position for La that maybe I'm not taking into account
or misplaced?
I've already checked the common problems in the calculations (ghost
bands, high QTL values, spheres overlapping, leaking core charge,
oscillation in energy/charge/forces, etc, etc,) but both sets of
calculations seems reasonably good. Any comment to this issue will be
highly appreciated.

The used numerical parameters were the following:
Rmt(La)=2.40
Rmt(Co)=1.90
Rmt(O)=1.65
RmtxKmax=7.0
Lmax=6
GMAX=16
kpoints=14x14x14
mixing=0.1 (case.inm "0.10  1.00      PW and CLM-scaling factors")
force convergence= 0.1 (case.inM:  "PORT 0.1")
energy cutoff=-7.7 Ry
and no smearing was used (case.in2 "TETRA    0.000")

Regards,

Omar De la Peña Seaman

-- 
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Karlsruher Institut für Technologie (KIT)
Institut für Festkörperphysik

Omar De la Peña Seaman PhD
Posdoctorant
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