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[pleu@stanford.edu]

Hi,
     I am trying to calculate the GW bandgap correction to an Si slab, 
but the calculations do not seem to converge with increasing vacuum 
size.  The vacuum space has been tried at 10 A up to about 40A and the 
bandgap seems to increase linearly with all other parameters held the 
same.  
     What am I missing here?    
     Here is the input file below.  Thanks    




# 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   100         # Number of bands in KSS file (-1 means the 
maximum possible)
nband1      63         # Number of (occ and empty) bands to be computed
istwfk1     10*1

# Calculation of the screening (epsilon^-1 matrix)
optdriver2  3        # Screening calculation
getkss2     -1       # Obtain KSS file from previous dataset
nband2      126       # Bands to be used in the screening calculation
ecutwfn2    3      # Cut-off energy of the planewave set to represent 
the wavefunctions
ecuteps2    4      # 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
getkss3     -2       # Obtain KSS file from dataset 1
getscr3     -1       # Obtain SCR file from previous dataset
nband3      315       # Bands to be used in the Self-Energy calculation
ecutwfn3     3.0     # Planewaves to be used to represent the 
wavefunctions
ecutsigx3     4.0     # Dimension of the G sum in Sigma_x
                     # (the dimension in Sigma_c is controlled by 
npweps)
nkptgw3      1                # number of k-point where to calculate 
the GW correction
kptgw3                       # k-points
  0.000    0.000    0.000
bdgw3         5  6

# Data common to the three different datasets
#Definition of the unit cell 
acell 1 1 1 Angstr 
rprim   3.8657499999999998  0.0000000000000000  0.0000000000000000 
        1.9328799999999999  3.3478400000000001  0.0000000000000000 
        0.0000000000000000  0.0000000000000000 41.5000000000000000 

# Definition of the atom types 
ntypat 2 # Number of types of atom
znucl 14  1 
                  # 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 4 # There are two atoms 
typat  2*1 2*2 
xcart              # Cartesian coordinate of atoms 
  1.93288  1.11595  2.26016 
  0.00000  0.00000  1.51168 
  1.93288  1.11595  3.77030 
  0.00000  0.00000  0.00000  Angstr 

# Definition of the k-point grid
kptopt  1            # Option for the automatic generation of k points,
nkpt    10
ngkpt  4 4 1  
nshiftk 1
shiftk  0.0 0.0 0.0  # These shifts will be the same for all grids

#Definition of the planewave basis set 
ecut 245.435 eV         # Maximal kinetic energy cut-off, in eV 

# 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.