Hi J?rg, 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, J?rg 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,*>* J?rg*>**>**>* 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 Qu?mica Fisica Aplicada Facultad de Ciencias Universidad Aut?noma 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 conocer? y que a su vez padecer?n, esperar?n y trabajar?n para otros, que tampoco ser?n felices, pues el hombre ans?a siempre una felicidad situada m?s all? de la porci?n que le es otorgada. Pero la grandeza del hombre est? precisamente en querer mejorar lo que es. En imponerse Tareas. En el Reino de los Cielos no hay grandeza que conquistar, puesto que all? todo es jerarqu?a establecida, inc?gnita despejada, existir sin t?rmino, 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 m?xima medida en el Reino de este Mundo*". Alejo Carpentier, El reino de este mundo, (1949). Translate from spanish here http://translate.google.com/?hl=&ie=UTF-8&text=&sl=es&tl=en -------------- next part -------------- An HTML attachment was scrubbed... URL: http://www.democritos.it/pipermail/pw_forum/attachments/20111209/9ec46c3d/attachment.htm
