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[PGanesh <pganesh@ciw.edu>]

Dear All,

I find that the piezo constants (of say AlAs as in tutorial on 
non-linear properties) vary depending on how the 'ddk' has been 
calculated.  If I use "tffield_2.in" as the input, which uses the 
"berryopt -2" to get 'ddk' as given in tutorial on non-linear 
properties, then I get the piezo. (relaxed ion, C/m^2) to be:

     0.00000000      0.00000000      0.00000000
     0.00000000      0.00000000      0.00000000
     0.00000000      0.00000000      0.00000000
     0.04574986      0.00000000      0.00000000
     0.00000000      0.04574976      0.00000000
     0.00000000      0.00000000      0.04574972


But if I compute 'ddk' using "rfelfd = 2" instead of doing "berryopt = 
-2", then I get  piezo (relaxed ion, C/m^2) to be:

     0.00000000      0.00000000      0.00000000
     0.00000000      0.00000000      0.00000000
     0.00000000      0.00000000      0.00000000
     0.33893266      0.00818336     -0.00672708
    -0.00145628      0.32155178      0.00672708
    -0.00145628     -0.00818336      0.26525465

Can someone explain which is the right input to compute piezo. contsants 
and which is the correct piezo since the value in the website is 
different from both?  I am using v5.4.2.  

Thanks,
PGanesh

PS: The input file for the second calculation of piezo. is pasted 
below.  Additions and omissions w.r.t default input have been indicated.

# Linear response calculation for AlAs
# Perturbation: atomic displacements & strains
# Finite difference calculation of the ddk
# (M. Veithen, 28.4.2005)

#Definition of lattice parameters
#********************************
  acell       3*10.53
  rprim       0.0 0.5 0.5
              0.5 0.0 0.5
              0.5 0.5 0.0

#Definition atomic positions and types
#*************************************
  natom       2
  ntypat      2
  znucl       13 33
  typat       1 2
  xred        0    0    0
              0.25 0.25 0.25
  ixc         3

#Parameters of the SCF cycles
#****************************
  iscf        5
  nstep       100


#Pland wave basis and k-point grid
#*********************************
  ecut        2.8
  ecutsm      0.5
  dilatmx     1.05
  ngkpt       6 6 6
  nshiftk     4
  shiftk      0.5 0.5 0.5
              0.5 0.0 0.0
              0.0 0.5 0.0
              0.0 0.0 0.5

ndtset 3     jdtset 1 2 3

#DATASET1 : scf calculation: GS WF in the BZ
#********************************************
  prtden1    1
  kptopt1    1
  toldfe1    1.0d-12

#DATASET2 : non scf calculation: GS WF in the whole BZ
#*****************************************************
  getden2    1
  kptopt2    2
    iscf2   -3
  getwfk2    1
  tolwfr2    1.0d-22
   nband2    4
# berryopt2   -2                       # New omission
   rfelfd2 2                               #New addition
   rfdir2    1 1 1

#DATASET3 : linear response to atomic displacements and strains
#**************************************************************
  getwfk3    2
  getddk3    2
   rfdir3    1 1 1
  rfstrs3    3
  rfphon3    1
 rfatpol3    1 2
  toldfe3    1.0d-12
   nband3    4
  kptopt3    2
 rfelfd3 3                                   # New addition
nbdbuf 0