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["Hua Bao" <hbao@purdue.edu>]

Title: CNT optic calculation fails at larger box?

Dear All,

I am trying to calculated the dielectric function of carbon nanotube (CNT) using abinit and optic. A 3D box is constructed, and the CNT axis is along the Z direction. I calculated the optical dielectric function using different box size: the xy box is chosen to be 10*10 angstrom (A) and 15*15 angstrom. Since the (5,0) tube diameter is around 4 A, the 6 A vacuum is enough so that the tube-tube interaction is minimal. However, for the second box (15*15 A), I can bearly see any the peaks, which is in the smaller box calculation. The calculated peaks in small box is consistent with exist literatures . However, the peaks around 1.5 eV and 2.5 eV are completely gone for the larger box. (See the attached figure, the purple curve is for the larger box)

I know there is a term in the Fermi's golden rule equation to calculate the imaginary part of the dielectric function. The epsilon2 (imaginary part) is inverse proportional to the box volume. However, I don't think that is the reason to cause such a large difference between the calculated dielectric function as the figure shows.

<<...>>

By the way, carefully convergence study has been performed before the optic calculating. The kpoint grid, cutoff, and box size should be enough for a calculation like this. Also, the calculated band structure of the two cases are also similar.

Thanks,
Hua

Here are the input files for preparing wave function and optic calculation:
*******************************************************************************************
 ndtset 6

#First dataset : SC run with kpoints in the IBZ
  nband1  40
  nstep1 200
 kptopt1 1
 nbdbuf1 0
 prtden1 1   getden1 0   getwfk1 0    ! Usual file handling data

#Second dataset : NSC run with large number of bands, and points in the IBZ
    iscf2 -2
   nband2 120  ! Minimal number of bands for linear optics (imaginary part of the spectrum)
   nstep2 200
  kptopt2  1
  getwfk2  1   getden2 1   ! Usual file handling data

#Third dataset : NSC run with large number of bands, and points in the the full BZ
    iscf3 -2
   nband3 120  ! Minimal number of bands for linear optics (imaginary part of the spectrum)
   nstep3 200
  kptopt3  2  ! Time-reversal symmetry can be used in the present implementation for linear optics
  getwfk3  2   getden3 1   ! Usual file handling data

#Fourth dataset : ddk response function along axis 1
   iscf4 -3
  nband4  120   ! Minimal number of bands for linear optics (imaginary part of the spectrum)
  nstep4  1
  nline4  0
 kptopt4  2   ! Time-reversal symmetry can be used in the present implementation for linear optics

   nqpt4  1  qpt4  0.0d0 0.0d0 0.0d0
  rfdir4  1 0 0
 rfelfd4  2
 getwfk4  3

#Fifth dataset : ddk response function along axis 2
   iscf5 -3
  nband5  120   ! Minimal number of bands for linear optics (imaginary part of the spectrum)
  nstep5  1  nline5  0
 kptopt5  2   ! Time-reversal symmetry can be used in the present implementation for linear optics

   nqpt5  1  qpt5  0.0d0 0.0d0 0.0d0
  rfdir5  0 1 0
 rfelfd5  2
 getwfk5  3

# Sixth dataset : ddk response function along axis 3
   iscf6 -3
  nband6  120   ! Minimal number of bands for linear optics (imaginary part of the spectrum)
  nstep6  1  nline6  0
 kptopt6  2   ! Time-reversal symmetry can be used in the present implementation for linear optics

   nqpt6  1  qpt6  0.0d0 0.0d0 0.0d0
  rfdir6  0 0 1
 rfelfd6  2
 getwfk6  3

#Data common to all datasets
 ngkpt  1 1 24      ! This is much too low : should be at least 24x24x24

nshiftk 1
shiftk -0.5 -0.5 0

 acell 15 15 4.2388 angstrom
! amu 69.72  74.9216
 ecut 30.00
! pawecutdg 40.0
! iscf 3
 natom  20
 ntypat  1
! rprim   0 .5 .5  .5 0 .5  .5 .5 0

xangst        2.0109995638E+00  0.0000000000E+00  3.1687260048E-02
              1.6507945612E+00  1.1971880202E+00  3.5603483549E+00
              1.6510773880E+00  1.1972294023E+00  2.1641332662E+00
              6.2216556189E-01  1.9228680361E+00  1.4398196864E+00
              6.2193346632E-01  1.9223535306E+00  3.4751819953E-02
             -6.2193346632E-01  1.9223535306E+00  3.5652481800E+00
             -6.2216556189E-01  1.9228680361E+00  2.1601803136E+00
             -1.6510773880E+00  1.1972294023E+00  1.4358667338E+00
             -1.6507945612E+00  1.1971880202E+00  3.9651645132E-02
             -2.0109995638E+00  0.0000000000E+00  3.5683127400E+00
             -2.0110928000E+00  0.0000000000E+00  2.1576060198E+00
             -1.6510773880E+00 -1.1972294023E+00  1.4358667338E+00
             -1.6507945612E+00 -1.1971880202E+00  3.9651645132E-02
             -6.2193346632E-01 -1.9223535306E+00  3.5652481800E+00
             -6.2216556189E-01 -1.9228680361E+00  2.1601803136E+00
              6.2216556189E-01 -1.9228680361E+00  1.4398196864E+00
              6.2193346632E-01 -1.9223535306E+00  3.4751819953E-02
              1.6507945612E+00 -1.1971880202E+00  3.5603483549E+00
              1.6510773880E+00 -1.1972294023E+00  2.1641332662E+00
              2.0110928000E+00  0.0000000000E+00  1.4423939802E+00

 typat  1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1
 tolwfr  1.e-16
 znucl  6
 chkprim 0

*********************************************************************************
toptic_3o_DS4_1WF61
toptic_3o_DS5_1WF62
toptic_3o_DS6_1WF63
toptic_3o_DS3_WFK
0.001         ! Value of the smearing factor, in Hartree
0.00005  0.2   ! Difference between frequency values (in Hartree), and maximum frequency ( 1 Ha is about 27.211 eV)
0.000         ! Scissor shift if needed, in Hartree
0.002         ! Tolerance on closeness of singularities (in Hartree)
1             ! Number of components of linear optic tensor to be computed
33            ! Linear coefficients to be computed (x=1, y=2, z=3)
0             ! Number of components of nonlinear optic tensor to be computed
              ! Non-linear coefficients to be computed

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