The ABINIT Users Mailing List ( CLOSED )

[Fabien Bruneval <fabien.bruneval@cea.fr>]

No, no it's not that wierd.

The problem comes from the plasmon-pole model that assumes the 
dielectric function to be a delta function in frequency.
Integrating numerically a delta function is indeed very difficult for 
the code and therefore produces unrealistic values.

As a rule of thumb you can use:
If you are interested in states that are not within a plasmon frequency 
to the band gap, you must switch off the plasmon pole model.

Please follow the tutorial to get the full calculations. The main 
keyword is gwcalctyp=2.


Good luck


Fabien




On 10/06/2009 09:34 AM, matthieu verstraete wrote:
> You are (1) using a plasmon pole model, and (2) doing perturbative GW,
> so there is formally no guarantee for corrections far from the Fermi
> level.
>
> Nevertheless, you are right, this is wierd, and looks like it is all
> coming from the renormalization Z which is huge for just that state.
> It is also possible that you need many more bands or a different
> frequency treatment (eg contour deformation) to get higher energies
> right, as the screening has to be correct out to higher energies for
> sigma to be ok. Have you checked how high in band index and energy the
> corrections in Hybertsen+Louie go?
>
> Matthieu
>
> On Tue, Oct 6, 2009 at 8:49 AM, Alexey Kosobutsky<kosobutsky@list.ru>  wrote:
>    
> > Dear Abinit users,
> >
> > I am interested in the calculation of the correct position of Si conduction
> > band levels.
> > For lower conduction bands the results of GW calculation (obtained with
> > using of Abinit 5.8.4) seem to be reasonable, but qusiparticle corrections
> > for levels starting from 11-12 are rather strange: many of them are negative
> > with large absolute values.
> > For example, for k-point (-0.125, -0.250, 0.000) from a 4x4x4 mesh I have
> > obtained the following result:
> >
> > k =   -0.125  -0.250   0.000
> > Band     E0<VxcLDA>    SigX SigC(E0)      Z dSigC/dE  Sig(E)    E-E0       E
> >     1  -5.418 -10.505 -17.059   6.668   0.597  -0.676 -10.437   0.068  -5.350
> >     2   2.652 -10.845 -13.432   2.000   0.744  -0.344 -11.281  -0.437   2.215
> >     3   4.563 -10.815 -12.460   1.081   0.762  -0.312 -11.246  -0.431   4.132
> >     4   4.641 -10.851 -12.328   0.940   0.763  -0.310 -11.261  -0.410   4.231
> >     5   7.998  -9.855  -5.904  -3.727   0.775  -0.290  -9.681   0.174   8.172
> >     6   9.579 -10.423  -5.761  -4.418   0.762  -0.312 -10.238   0.185   9.764
> >     7  10.380 -10.413  -5.576  -4.581   0.758  -0.320 -10.220   0.194  10.574
> >     8  10.627  -9.563  -4.653  -4.594   0.767  -0.303  -9.320   0.242  10.869
> >     9  13.367 -10.468  -4.758  -5.488   0.737  -0.358 -10.304   0.164  13.531
> >    10  14.235  -9.918  -4.040  -5.494   0.741  -0.350  -9.634   0.284  14.520
> >    11  15.745  -8.530  -2.655  -5.884   0.722  -0.385  -8.536  -0.007  15.739
> >    12  17.387  -8.595  -2.469  -6.227   0.695  -0.438  -8.666  -0.070  17.317
> >    13  19.090  -9.078  -2.448  -6.954   0.654  -0.528  -9.290  -0.212  18.878
> >    14  20.359  -9.098  -2.371  -7.261   0.623  -0.605  -9.431  -0.332  20.027
> >    15  23.609  -9.946  -2.666  -9.303   0.432  -1.316 -10.820  -0.874  22.735
> >    16  23.992  -9.955  -2.619  -9.848   0.464  -1.157 -11.120  -1.165  22.827
> >    17  25.101  -8.495  -1.678  -9.661   0.720  -0.389 -10.543  -2.048  23.053
> >    18  27.486 -10.036  -2.321  -8.687   2.669   0.625 -12.629  -2.592  24.894
> >    19  27.716 -10.426  -2.445  -9.965   8.090   0.876 -26.477 -16.051  11.665
> >    20  29.017 -10.024  -2.232  -8.679   1.023   0.023 -10.932  -0.908  28.109
> >    21  32.152 -10.437  -2.111  -8.320   1.730   0.422 -10.426   0.011  32.163
> >    22  33.120 -10.525  -2.214  -8.377   0.810  -0.234 -10.578  -0.053  33.067
> >    23  34.348 -10.485  -2.040  -7.689   1.144   0.126  -9.619   0.866  35.214
> >    24  35.686  -9.563  -1.642  -7.270   1.029   0.029  -8.892   0.671  36.357
> >    25  37.497  -9.407  -1.464  -6.995   1.005   0.005  -8.454   0.953  38.450
> >    26  38.027 -10.288  -1.976  -6.171   1.425   0.298  -7.238   3.050  41.077
> >    27  38.070  -9.253  -1.467  -6.888   1.528   0.345  -7.881   1.372  39.442
> >    28  39.153  -9.729  -1.659  -5.469   0.687  -0.455  -7.941   1.788  40.941
> >    29  39.671  -9.579  -1.471  -6.684   0.580  -0.725  -8.753   0.826  40.497
> >    30  40.535  -9.818  -1.569  -6.329   0.846  -0.182  -8.194   1.624  42.159
> >
> > As one can see, the QP correction E-E0 can be as large as -16 eV (for 19th
> > band) that looks quite unphysically. The content of input file used for
> > calculation is shown below, it is modified tgw1_1.in file with sufficiently
> > high (as I think) values of convergence controlling parameters. The
> > corresponding log file is attached, the majority of warnings notify about
> > "Values of Re Sig_c are not linear". The question is: how to calculate high
> > levels of conduction band with sufficient accuracy?
> >
> > Best wishes,
> > Alexey
> >
> >
> > Input file:
> > # Crystalline silicon
> > # Calculation of the GW corrections
> > # Dataset 1: ground state calculation and of the kss file for 10 k-points in
> > IBZ
> > # Dataset 2: calculation of the screening (epsilon-1 matrix for W)
> > # Dataset 3: calculation of the Self-Energy matrix elements (GW corrections)
> >
> > ndtset      3
> >
> > # Definition of parameters for the calculation of the KSS file
> > nbandkss1   -1         # Number of bands in KSS file (-1 means the maximum
> > possible)
> > nband1       9         # Number of (occ and empty) bands to be computed
> > #istwfk1     10*1
> >
> > # Calculation of the screening (epsilon-1 matrix)
> > optdriver2  3        # Screening calculation
> > gwpara2     2
> > getkss2     -1       # Obtain KSS file from previous dataset
> > nband2      54       # Bands to be used in the screening calculation
> > ecutwfn2    4.0      # Cut-off energy of the planewave set to represent the
> > wavefunctions
> > ecuteps2    8.0      # Cut-off energy of the planewave set to represent the
> > dielectric matrix
> > ppmfrq2    16.7 eV  # Imaginary frequency where to calculate the screening
> >
> > # Calculation of the Self-Energy matrix elements (GW corrections)
> > optdriver3  4        # Self-Energy calculation
> > gwpara3     1
> > getkss3     -2       # Obtain KSS file from dataset 1
> > getscr3     -1       # Obtain SCR file from previous dataset
> > nband3       150       # Bands to be used in the Self-Energy calculation
> > ecutwfn3     8.0     # Planewaves to be used to represent the wavefunctions
> > ecutsigx3    8.0     # Dimension of the G sum in Sigma_x
> >                      # (the dimension in Sigma_c is controlled by npweps)
> > nkptgw3      2               # number of k-point where to calculate the GW
> > correction
> > kptgw3                       # k-points
> >    -0.125  -0.250   0.000
> >    -0.125   0.250   0.000
> > bdgw3    1 30  1 30           # calculate GW corrections for 2 k-points for
> > bands from 1 to 30
> >
> > # Data common to the three different datasets
> >
> > # Definition of the unit cell: fcc
> > acell  3*10.217        # This is equivalent to   10.217 10.217 10.217
> > rprim  0.0  0.5  0.5   # FCC primitive vectors (to be scaled by acell)
> >         0.5  0.0  0.5
> >         0.5  0.5  0.0
> >
> > # Definition of the atom types
> > ntypat  1         # There is only one type of atom
> > znucl 14          # The keyword "znucl" refers to the atomic number of the
> >                   # possible type(s) of atom. The pseudopotential(s)
> >                   # mentioned in the "files" file must correspond
> >                   # to the type(s) of atom. Here, the only type is Silicon.
> >                         # Definition of the atoms
> > natom 2           # There are two atoms
> > typat  1 1        # They both are of type 1, that is, Silicon.
> > xred              # Reduced coordinate of atoms
> >       0.0  0.0  0.0
> >       0.25 0.25 0.25
> >
> >
> > # Definition of the k-point grid
> > kptopt  1            # Option for the automatic generation of k points,
> > nkpt    10
> > ngkpt   4 4 4
> > nshiftk 4
> > shiftk  0.5 0.5 0.5  # These shifts will be the same for all grids
> >         0.5 0.0 0.0
> >         0.0 0.5 0.0
> >         0.0 0.0 0.5
> > istwfk  10*1         # This is mandatory in all the GW steps.
> >
> > # Use only symmorphic operations
> > symmorphi 0
> >
> > # Definition of the planewave basis set (at convergence 16 Rydberg 8
> > Hartree)
> > ecut 8.0          # Maximal kinetic energy cut-off, in Hartree
> >
> > # Definition of the SCF procedure
> > nstep   10        # Maximal number of SCF cycles
> > toldfe  1.0d-6    # Will stop when this tolerance is achieved on total
> > energy
> > diemac  12.0      # Although this is not mandatory, it is worth to
> >                   # precondition the SCF cycle. The model dielectric
> >                   # function used as the standard preconditioner
> >                   # is described in the "dielng" input variable section.
> >                   # Here, we follow the prescription for bulk silicon.
> >
> >
> > # This line added when defaults were changed (v5.3) to keep the previous,
> > old behaviour
> >   iscf 5
> >
> >
> >
> >      
>
>
>