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[Xavier Gonze <gonze@pcpm.ucl.ac.be>]

Dear Gustavo,

You definitely need to use a non-zero value of   ecutsm
to have converged results at a much lower ecut ...
Why did you set it explicitely to zero ?

Xavier


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