[Pw_forum] CaCuO2 and LDA+U
matteo at umn.edu
Tue Mar 25 16:04:15 CET 2008
Dear Chao Cao,
I'm not sure that I can provide very useful hints for this problem. with
the few informations you provide it's a little bit like shooting in the
First of all are you sure you are doing equivalent calculations with
Vasp and PWscf? for example, are cutoffs the same? the smearing? what
about potentials? Of course, qualitatively, results should not depend
too much on the pseudopotential. But the story is a little bit different
for LDA+U calculations. In some cases with different pseudos you may
need quite different Us to obtain the same results (e.g. as large as 1
eV differences may be needed switching from LDA to GGA). Did you try to
"play" a little bit with U and check what happens for bigger values?
I checked your input file and it looks fine to me. However you use a
curious unit cell that is rotated 45 degrees wrt the cartesian axes on
the xy plane. Again this shouldn't matter in standard calculations.
However with LDA+U sometimes the code gets trapped in local minima and
the initial choice of occupations could be important. changing the
orientation of the unit cell will change the orientation of the
spherical harmonics used to obtain the atomic occupations. Maybe this
will help finding the right ground state.
Apart from this Cu compounds may be problematic (especially with oxygen
present). I have no direct experience yet but you can try to look for
Paul Grant's email to the forum on the topic. How many electrons do you
get on the Cu atoms?
Hope this helps.
Chao Cao wrote:
> Dear PWSCF users and developers:
> I have been trying to use PWSCF's implementation of LDA+U to calculate
> the electronic structure of CaCuO2 lately. In VASP, once I turn on U, it
> gives AFM ground state and reasonable gap separation at E_F. However,
> the procedure seems to be much harder for PWSCF. While it gets AFM gs
> correctly, I can never get a band gap. I searched the forum, and I
> realized that the initial occupation number might be important, but the
> several combinations I tried seem to give me the same result. So, is
> there any hint or suggestions, especially how to choose initial
> occupation numbers? Thanks in advance.
> Chao Cao
> And, here's my input file:
> calculation = 'scf'
> restart_mode = 'from_scratch'
> prefix = 'CaCuO2'
> disk_io = 'low'
> outdir = './'
> ibrav = 0, celldm(1) = 1.88972688, ntyp = 4, nat = 16
> nbnd=80, ecutwfc = 32, ecutrho = 400
> nspin = 2, starting_magnetization(2)=0.5, starting_magnetization(3)=-0.5
> lda_plus_u = .true., Hubbard_U(2) = 6.5, Hubbard_U(3) = 6.5
> occupations = 'smearing', smearing = 'mp', degauss = 0.007
> mixing_beta = 0.2
> conv_thr = 1.0d-9
> Ca 1.0 Ca.pbe-nsp-van.UPF
> Cuu 1.0 Cu.pbe-n-van.UPF
> Cud 1.0 Cu.pbe-n-van.UPF
> O 1.0 O.pbe-van.UPF
> ATOMIC_POSITIONS crystal
> Ca 0.25 0.75 0.25
> Ca 0.75 0.25 0.25
> Ca 0.25 0.75 0.75
> Ca 0.75 0.25 0.75
> Cuu 0.25 0.25 0.00
> Cuu 0.75 0.75 0.50
> Cud 0.75 0.75 0.00
> Cud 0.25 0.25 0.50
> O 0.00 0.00 0.00
> O 0.50 0.00 0.00
> O 0.00 0.50 0.00
> O 0.50 0.50 0.00
> O 0.00 0.00 0.50
> O 0.50 0.00 0.50
> O 0.00 0.50 0.50
> O 0.50 0.50 0.50
> K_POINTS automatic
> CELL_PARAMETERS cubic
> 3.878393250 3.878393249 0.000000000
> -3.878393249 3.878393250 0.000000000
> 0.000000000 0.000000000 6.409031058
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