Dear Prof. Blaha and Prof. Fecher,
Thank you for kind advices. I tried to increase the convergence parameter and had a good result! If it is not enough in another compound, I will also try to increase the data point from 5 to 7. Best Regards, Izumi Hase tel: 029-861-5147 fax: 029-861-5569 e-mail: i.h...@aist.go.jp ________________________________ 差出人: Wien <wien-boun...@zeus.theochem.tuwien.ac.at> が Fecher, Gerhard <fec...@uni-mainz.de> の代理で送信 送信日時: 2017年3月28日 16:13:12 宛先: A Mailing list for WIEN2k users 件名: Re: [Wien] Question about ELAST Just a reminder, the elastic constants are calculated from second derivatives of the energy, therefore, the energy needs to be well converged also with respect to k points and plane waves. As Peter told, the RMS=0 sounds tempting, however, the error of a fitted parameter is proportional to 1/(N-n) where N is the number of data to be fitted and n the number of fitted parameters, here it becomes 1/(N-n) = 1/(5-5) = 1/0, that is, the error is proportional to infinite ! I would use a third order fit with 7 energies (distortions) and then check the changes when using fourth order. Indeed, it should be possible to use statistics for an error estimation for the second order parameter that gives the elastic constants (or a combination of elastic constants). Ciao Gerhard DEEP THOUGHT in D. Adams; Hitchhikers Guide to the Galaxy: "I think the problem, to be quite honest with you, is that you have never actually known what the question is." ==================================== Dr. Gerhard H. Fecher Institut of Inorganic and Analytical Chemistry Johannes Gutenberg - University 55099 Mainz and Max Planck Institute for Chemical Physics of Solids 01187 Dresden ________________________________________ Von: Wien [wien-boun...@zeus.theochem.tuwien.ac.at] im Auftrag von Peter Blaha [pbl...@theochem.tuwien.ac.at] Gesendet: Dienstag, 28. März 2017 07:59 An: A Mailing list for WIEN2k users Betreff: Re: [Wien] Question about ELAST Hi, One guess: The energy changes are very small. It might be,that the default convergence (-ec 0.0001) is not sufficient to give energies with the required accuracy. Rerun the scripts, but increase the convergence parameter in the run_lapw statements. PS: When you have 5 data points and make a polynomial fit of order 4, the fit must go through the data points giving RMS=0. But this does NOT mean that your calculations are ok. Trust more a 3rd order fit !!! Am 24.03.2017 um 11:47 schrieb 長谷泉: > Dear Wien2k Users, > > > Hello, I have a question about ELAST, which calculates the elastic > constants for cubic lattice. > > I made a test calculation for NaCl (fcc). I used default settings > (#k=1000, Vxc=PBE, RKmax=7.0, rmt=2.5 for Na and Cl). > > And I used the default values for eos/tetra/rhomb.job . > > > For eos I got reasonable value (B_cal=29.7GPa while B_exp=24.4GPa). > However, for tet and rhomb I got very abnormal values. I obtained > unphysical C44<0, while C44_exp=12.6GPa. > > > Especially for tet, E(x) is too soft, not harmonic, and even not symmetric. > > I show tetra.output as follows: > > > Polynomial fit or tetragonal strain done > > A RMS of 0.203369E-12 was achieved using a polynome of degree : 4 > > > At volume= 301.07 bohr^3 > > C11-C12 is: 0.000062 a.u or 0.915 GPa > > > Strain energy dE > > 0.00675692 -1248.135736 0.227374E-12 > > 0.01370014 -1248.135102 0.227374E-12 > > -0.00000019 -1248.135686 0.227374E-12 > > -0.00657965 -1248.135568 0.227374E-12 > > -0.01298805 -1248.135200 0.000000E+00 > > > I changed the lattice constant slightly, and for P~0.1GPa I got almost > the same result. > > > > As for MgO which has the same crystal structure with NaCl, I obtained > almost the same values as the UsersGuide by T.Charpin, 12th July 2001. > > > Any kind of help will be greatly appreciated. > > > Best Regards, > > > > Izumi Hase > > National Institute of Advanced Industrial Science and Technology (AIST), > Japan > > > tel: 029-861-5147 fax: 029-861-5569 > > e-mail: i.h...@aist.go.jp > > > > > _______________________________________________ > Wien mailing list > Wien@zeus.theochem.tuwien.ac.at > http://zeus.theochem.tuwien.ac.at/mailman/listinfo/wien > SEARCH the MAILING-LIST at: > http://www.mail-archive.com/wien@zeus.theochem.tuwien.ac.at/index.html > -- -------------------------------------------------------------------------- Peter BLAHA, Inst.f. Materials Chemistry, TU Vienna, A-1060 Vienna Phone: +43-1-58801-165300 FAX: +43-1-58801-165982 Email: bl...@theochem.tuwien.ac.at WIEN2k: http://www.wien2k.at WWW: http://www.imc.tuwien.ac.at/staff/tc_group_e.php -------------------------------------------------------------------------- _______________________________________________ Wien mailing list Wien@zeus.theochem.tuwien.ac.at http://zeus.theochem.tuwien.ac.at/mailman/listinfo/wien SEARCH the MAILING-LIST at: http://www.mail-archive.com/wien@zeus.theochem.tuwien.ac.at/index.html _______________________________________________ Wien mailing list Wien@zeus.theochem.tuwien.ac.at http://zeus.theochem.tuwien.ac.at/mailman/listinfo/wien SEARCH the MAILING-LIST at: http://www.mail-archive.com/wien@zeus.theochem.tuwien.ac.at/index.html
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