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["D. R. Hamann" <drh@bell-labs.com>]

Dear Gustavo,

I think your symmetry problem comes from the fact that you are defining your
lattice vectors for fcc with angdeg instead of inputing rprim yourself.
This probably gives you a rhombohedral set of rprim, which will not be
aligned with the cartesian axes in a manner which will produce the
symmetries you expect.  I suggest trying

rprim  0.0 0.5 0.5
       0.5 0.0 0.5
       0.5 0.5 0.0

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

In terms of the ecut convergence, I don't think that Xavier's suggestion
about ecutsm is the issue.  While necessary for lattice constant optimization,
and for elastic constants by numerical differentiation (unless you use very
small strains), I've found that as presently implemented, nonzero ecutsm
makes elastic constant convergence worse.

Are you using the 11na.9.hgh potential?  If so, the semicore states probably
require a very large cutoff, although 400 seems rather extreme.

Don Hamann

gdb@quimica.unp.edu.ar wrote:

Dear All,

I am trying to use the new features in abinit-4.3.3 to calculate the elastic constants of NaCl using the HGH pseudopotentials. I have done several runs using the input files given below.

Though it seems that ngkpt= 4 4 4 gives reasonably converged values with respect to the number of k points, the ecut values to get convergece seem to be extremly large: more than 400 Ha !!!! 

On the other hand, by simmetry C12 has to be equal to C13, and C44=C66. However this is not what I have obtained from my calculations. Though these conditions may not be fulfilled for low values of ngkpt, I can not see the situation improving as ngkpt is increased

The questions then are:
1) why is it necesary such a extremly large ecut? 
2) why the conditions C12=C13 and C44=C66 do not hold?


Hope I am making just some simple mistake in the input files

Thanks

Gustavo Barrera


NaCl elastic constants
C11
ecut/ nkpt 2	4	6	8
40      528.2	517	516.6	
80	467	455	454.2	454.2
200	89.5	78.1	77.8	
300	66.3	55.0		
400	65.2	53.8		

C12
ecut/ nkpt 2	4	6	8
40	-35.7	-38.0	-38.1	
80	66.8	64.7	64.6	64.7
200	26.3	24.2	24.1	
300	20.4	18.3		
400	20.1	17.9		

C13
ecut/ nkpt 2	4	6	8
40	-34.5	-33.3	-33.4	
80		69.1	69.2	69.2
200	28.9	29.0	29.1	
300	23.0	23.2		
400	22.7	22.8		

C44
ecut/ nkpt 2	4	6	8
40	283.2	282.0	281.9	
80	202	199.4	199.4	199.3
200	34.1	31.9	31.8	
300	25.5	23.3		
400	25.1	22.9		

C66
ecut/ nkpt 2	4	6	8
40	281.9	277.3	277.2	
80		194.9	194.8	194.8
200	31.7	27.0	26.8	
300	23.0	18.4		
400	22.5	18.0		

##### My abinit input file:
# NaCl crystal
#

ndtset 3
# nband 8

#DATASET 1 : SC run
    nqpt1  0
  kptopt1  1
    iscf1  5
  tolvrs1  1.0d-18       # SCF stopping criterion


#DATASET 2 : ddk perturbation
 getwfk2   -1
   iscf2   -3
   nqpt2   1
    qpt2   0.0d0 0.0d0 0.0d0
  rfdir2   1 1 1
 rfelfd2   2
 kptopt2   2
 tolwfr2   1.0d-22


#DATASET 3 : phonons at gamma
  getddk3   -1
  getwfk3   -2
    nqpt3   1
     qpt3   0.0d0 0.0d0 0.0d0
 rfatpol3   1 2
   rfdir3   1 1 1
  rfelfd3   3
  rfphon3   1
  rfstrs3   3
  tolvrs3   1.0d-10
    iscf3   3
  kptopt3   2


#Definition of the unit cell
acell 3*3.871        angstrom # lengths of the primitive vectors
angdeg 3*60.0

#Definition of the atom types
ntypat 2           # There is only one type of atom
zatnum 11 17       # The keyword "zatnum" 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.

#Definition of the atoms
natom 2           # total number of atoms in unit cell
typat 1 2          # They both are of type 1, that is, Hydrogen
xred              # atom positions in REDuced coordinates
   0.0 0.0 0.0    #
   0.5 0.5 0.5    #

#Definition of the planewave basis set
  ecut 200.0         # Maximal plane-wave kinetic energy cut-off, in Hartree
ecutsm 0.0

#Definition of the k-point grid
  ngkpt  6 6 6         # Number of Grid points for K PoinTs generation
 shiftk  0 0 0

#Definition of the SCF procedure
nstep 100          # Maximal number of SCF cycles


###### My anaddb input file:
!the flags
dieflag 3  !the frequency dependent dielectric tensor is calculated
elaflag  3  !the flag for the elastic constant
piezoflag 3
instrflag 1  !the flag for the internal strain tensor

!the effective charge part
asr 1
chneut  1

! Wavevector list number 1

nph1l 1 !number of phonons in list 1
qph1l  0.0 0.0 0.0 1.0 !the Gamma point

! Wave vector list no. 2

nph2l 1  !number of phonons in list 2
qph2l 0.0 0.0 1.0 0.0  ! why here the normalization factor is 0

  

-- 
  

D. R. Hamann				Phone: 908-582-4454
Director, Theoretical Materials    	Fax:   908-582-4702
  Physics Research (retired)		email: drh@physics.bell-labs.com
Bell Laboratories 
Lucent Technologies
700 Mountain Ave, Room 1D-371
Murray Hill, NJ 07974-0636 USA