[Pw_forum] Al pseudopotential and elastic properties

[Eduardo Ariel Menendez Proupin]

Hi Jörg,
Not certainly with Al, but with other elements I have some experience
in elastic constants.
I think you are doing things fine, but anyway I have a few comments

1) Using Ultrasoft pseudos I recently needed to use a cutoff of 60 Ry
to obtain results converged and consistent
when calculated either the energy or the stress tensor. You are using
85 Ry, OK, but be carefull the pseudo is defined in a discrete grid
and
using a very high cutoff may introduce noise. You may consider also
using more k-points, specially to reproduce the tiny energy
differences with a strain of 0.001.
2) In my case, using a lower cutoff, the constants obtained from the
stress tensor were better than those obtained form the energy. This
trend is the opposite of yours, or you may have error cancellation.
3) A strain of 0.001 is really low, check that the plot of E vs strain
and stress vs strain is not noisy.
I guess that is way you have such low *etot_conv_thr and**
forc_conv_thr=1.0D-10*
4) Investigate how are the elastic constants measured (typically by
ultrasound) and what is the typical strain
involved in the measurements.

*5)
PS: I also tried to get C11 by the second derivative the Energy
fitting*>* quadratic function which gave me C11=75.6GPa but this would
assume that*>* the elastic constant is constant over the strain
range(which could be*>* showed that's not the case for small
strains(<0.01) using the*>* numerical second derivative).*
You may add cubic and quartic terms to the fit, and the second
derivative should become independent on the range if DFT calculation
is well converged..


On 12/8/11 11:48 AM, Jörg Buchwald wrote:
>* Hi,*>* does anyone has any experiences with the Al pseudopotentials and the*>* elastic constants?*>* I was testing Al.pbe-n-van.UPF with which I could reproduce the elastic*>* constant and the cohesive energy quite well, but using the output of*>* the stress matrix I get a way too high results for C11(>150GPa) at a*>* strain of 0.001. Appliying strains about>3%, I get results which are*>* quite relastic for C11(90-110 GPa). I thought that this has something*>* to do whith the kind of approximation, so I also created a ultrasoft*>* (PZ-)LDA potential with the vanderbilt code and tried the*>* normconserving Al.pz-vbc.UPF, which gave me similar results.*>* I applied the strain by stretching the cell vectors and performing a*>* 'relax'-simulation.*>* This is my input file for a rel. strain of 0.003:*>* ---*>*   &control*>*      calculation='relax'*>*      prefix='Al-test11',*>*      pseudo_dir='/home/jbuchw/espresso-4.3.2/pseudo'*>*      outdir = '/home/jbuchw/scratch',*>*      tstress=.true.*>*      disk_io='none'*>*      etot_conv_thr=1.0D-10*>*      forc_conv_thr=1.0D-10*>*   /*>*   &system*>*      ibrav=  0,*>*      celldm(1) =7.6271186*>*      nat=  4,*>*      ntyp= 1,*>*      ecutwfc = 85.0*>*      occupations='smearing'*>*      smearing='mp'*>*      degauss=0.0007*>*   /*>*   &electrons*>*      diagonalization='david'*>*      mixing_mode='plain'*>*      conv_thr=1.0D-10*>*   /*>*   &ions*>*     bfgs_ndim=3*>*   /*>*   &cell*>*   !  cell_dofree='yz'*>*   /*>* ATOMIC_SPECIES*>*   Al  26.981539  Al.pbe-n-van.UPF*>* ATOMIC_POSITIONS crystal*>*   Al 0.00 0.00 0.00*>*   Al 0.50 0.50 0.00*>*   Al 0.00 0.50 0.50*>*   Al 0.50 0.00 0.50*>* K_POINTS automatic*>*   14 14 14 0 0 0*>* CELL_PARAMETERS*>*     0.9995541042   0.000000000   0.000000000*>*     0.000000000   0.996564411   0.000000000*>*     0.000000000   0.000000000   0.996564411*>* --*>* Am I doing something wrong, or does it have something to do with the*>* pseudopotential?*>* Regards,*>* Jörg*>**>**>* PS: I also tried to get C11 by the second derivative the Energy fitting*>* quadratic function which gave me C11=75.6GPa but this would assume that*>* the elastic constant is constant over the strain range(which could be*>* showed that's not the case for small strains(<0.01) using the*>* numerical second derivative).*



-- 


Eduardo Menendez Proupin
Departamento de Química Fisica Aplicada
Facultad de Ciencias
Universidad Autónoma de Madrid
28049 Madrid, Spain
Phone: +34 91 497 6706

On leave from: Departamento de Fisica, Facultad de Ciencias, Universidad de
Chile URL: http://fisica.ciencias.uchile.cl/~emenendez

"*Padece, espera y trabaja para gentes que nunca conocerá y que a su vez
padecerán, esperarán y trabajarán para otros, que tampoco serán felices,
pues el hombre ansía siempre una felicidad situada más allá de la porción
que le es otorgada. Pero la grandeza del hombre está precisamente en querer
mejorar lo que es. En imponerse Tareas. En el Reino de los Cielos no hay
grandeza que conquistar, puesto que allá todo es jerarquía establecida,
incógnita despejada, existir sin término, imposibilidad de sacrificio,
reposo y deleite. Por ello, agobiado de penas y de Tareas, hermoso dentro
de su miseria, capaz de amar en medio de las plagas, el hombre puede hallar
su grandeza, su máxima medida en el Reino de este Mundo*".
Alejo Carpentier, El reino de este mundo, (1949).

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