Something confusing in the calculation of bilayer graphene

[Tianshu Jiang in Beijing]

 Hi everyone, 
 I have been trying to calculate the band structure of bilayer graphene 
using CP2k 4.1. The input file follow the tutorial band structure of 
graphene 
<https://www.cp2k.org/exercises:2016_uzh_cmest:band_structure_calculation?s[]=graphene> 
, and is listed in the following 

&GLOBAL
  PROJECT twoLayerGraphBS_z18
  RUN_TYPE ENERGY
  PRINT_LEVEL MEDIUM
&END GLOBAL

&FORCE_EVAL
  METHOD Quickstep
  &DFT
    BASIS_SET_FILE_NAME BASIS_MOLOPT
    POTENTIAL_FILE_NAME POTENTIAL

    &POISSON
      PERIODIC XYZ 
    &END POISSON
    &QS 
      EXTRAPOLATION USE_GUESS
    &END QS
    &SCF
      SCF_GUESS ATOMIC
      EPS_SCF 1.0E-6
      MAX_SCF 300 

      ADDED_MOS 2
      &SMEAR ON
        METHOD FERMI_DIRAC
        ELECTRONIC_TEMPERATURE [K] 300 
      &END SMEAR
      &DIAGONALIZATION
        ALGORITHM STANDARD
        EPS_ADAPT 0.01
      &END DIAGONALIZATION
      &MIXING
        METHOD BROYDEN_MIXING
        ALPHA 0.2
        BETA 1.5
        NBROYDEN 8
      &END MIXING
    &END SCF

    &XC
      &XC_FUNCTIONAL PBE
      &END XC_FUNCTIONAL
    &END XC
    &KPOINTS
      SCHEME MONKHORST-PACK 3 3 1
      SYMMETRY OFF
      WAVEFUNCTIONS REAL
      FULL_GRID .TRUE.
      PARALLEL_GROUP_SIZE 0
      &BAND_STRUCTURE
        ADDED_MOS 6
        FILE_NAME graph_z18.bs
        &KPOINT_SET
          UNITS CART_BOHR
          SPECIAL_POINT 0.0 0.0 0.0
          SPECIAL_POINT 1./2. 0.0 0.0
          SPECIAL_POINT 2./3. 1./3. 0.0
          SPECIAL_POINT 0.0 0.0 0.0
          NPOINTS 80  
        &END
      &END BAND_STRUCTURE
    &END KPOINTS
    &PRINT
        &E_DENSITY_CUBE ON
            STRIDE 1 1 1
        &END E_DENSITY_CUBE
    &END PRINT
  &END DFT
  &SUBSYS
    &CELL
      ABC [angstrom] 2.4612 2.4612 21.0
      ALPHA_BETA_GAMMA 90. 90. 60.
      SYMMETRY HEXAGONAL
      PERIODIC XYZ
      MULTIPLE_UNIT_CELL 1 1 1
    &END CELL
    &TOPOLOGY
      MULTIPLE_UNIT_CELL 1 1 1
    &END TOPOLOGY
    &COORD
      SCALED
      C 1./3. 1./3. 0.
      C 2./3. 2./3. 0.
      C 2./3. 2./3. 1./6.
      C 3./3. 3./3. 1./6.
    &END
    &KIND C
      ELEMENT C
      BASIS_SET TZVP-MOLOPT-GTH
      POTENTIAL GTH-PBE-q4
    &END KIND
  &END SUBSYS

&END FORCE_EVAL




And the band structure I plotted is as follows

<https://lh3.googleusercontent.com/-KWZPrTYUGBs/W0WoPcsrDaI/AAAAAAAAABE/cLd4uxyz8WA36prGTQDk24w8y_XR_sP-QCLcBGAs/s1600/bilayerGraph.png>
The k-point path I choose is : \gamma->M->K->\gamma. There should not exist 
band gap near the point K and the profile of the line near point K should 
be parabola. 
Does anyone know where is the problem in my calculation setting ?

Thanks for any kind response !

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