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<p>Dear Prof. Blaha and Prof. Fecher,</p>
<p><br>
</p>
<p>Thank you for kind advices. I tried to increase the convergence parameter and had a good result!</p>
<p>If it is not enough in another compound, I will also try to increase the data point from 5 to 7.</p>
<p><br>
</p>
<p>Best Regards,</p>
<p><br>
</p>
<p>Izumi Hase</p>
<p><br>
</p>
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<p></p>
<p>tel: 029-861-5147 fax: 029-861-5569</p>
<p>e-mail: i.hase@aist.go.jp</p>
<p><br>
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<div id="x_divRplyFwdMsg" dir="ltr"><font face="Calibri, sans-serif" color="#000000" style="font-size:11pt"><b>差出人:</b> Wien <wien-bounces@zeus.theochem.tuwien.ac.at> が Fecher, Gerhard <fecher@uni-mainz.de> の代理で送信<br>
<b>送信日時:</b> 2017年3月28日 16:13:12<br>
<b>宛先:</b> A Mailing list for WIEN2k users<br>
<b>件名:</b> Re: [Wien] Question about ELAST</font>
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<div class="PlainText">Just a reminder, the elastic constants are calculated from second derivatives of the energy,<br>
therefore, the energy needs to be well converged also with respect to k points and plane waves.
<br>
<br>
As Peter told, the RMS=0 sounds tempting, however, the error of a fitted parameter is<br>
proportional to 1/(N-n) where N is the number of data to be fitted and n the number of fitted parameters,<br>
here it becomes 1/(N-n) = 1/(5-5) = 1/0, that is, the error is proportional to infinite !<br>
I would use a third order fit with 7 energies (distortions) and then check the changes when using fourth order.<br>
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).
<br>
<br>
Ciao<br>
Gerhard<br>
<br>
DEEP THOUGHT in D. Adams; Hitchhikers Guide to the Galaxy:<br>
"I think the problem, to be quite honest with you,<br>
is that you have never actually known what the question is."<br>
<br>
====================================<br>
Dr. Gerhard H. Fecher<br>
Institut of Inorganic and Analytical Chemistry<br>
Johannes Gutenberg - University<br>
55099 Mainz<br>
and<br>
Max Planck Institute for Chemical Physics of Solids<br>
01187 Dresden<br>
________________________________________<br>
Von: Wien [wien-bounces@zeus.theochem.tuwien.ac.at] im Auftrag von Peter Blaha [pblaha@theochem.tuwien.ac.at]<br>
Gesendet: Dienstag, 28. März 2017 07:59<br>
An: A Mailing list for WIEN2k users<br>
Betreff: Re: [Wien] Question about ELAST<br>
<br>
Hi,<br>
<br>
One guess: The energy changes are very small. It might be,that the<br>
default convergence (-ec 0.0001) is not sufficient to give energies<br>
with the required accuracy.<br>
<br>
Rerun the scripts, but increase the convergence parameter in the<br>
run_lapw statements.<br>
<br>
PS: When you have 5 data points and make a polynomial fit of order 4,<br>
the fit must go through the data points giving RMS=0. But this does NOT<br>
mean that your calculations are ok.<br>
<br>
Trust more a 3rd order fit !!!<br>
<br>
<br>
Am 24.03.2017 um 11:47 schrieb 長谷泉:<br>
> Dear Wien2k Users,<br>
><br>
><br>
> Hello, I have a question about ELAST, which calculates the elastic<br>
> constants for cubic lattice.<br>
><br>
> I made a test calculation for NaCl (fcc). I used default settings<br>
> (#k=1000, Vxc=PBE, RKmax=7.0, rmt=2.5 for Na and Cl).<br>
><br>
> And I used the default values for eos/tetra/rhomb.job .<br>
><br>
><br>
> For eos I got reasonable value (B_cal=29.7GPa while B_exp=24.4GPa).<br>
> However, for tet and rhomb I got very abnormal values. I obtained<br>
> unphysical C44<0, while C44_exp=12.6GPa.<br>
><br>
><br>
> Especially for tet, E(x) is too soft, not harmonic, and even not symmetric.<br>
><br>
> I show tetra.output as follows:<br>
><br>
><br>
> Polynomial fit or tetragonal strain done<br>
><br>
> A RMS of 0.203369E-12 was achieved using a polynome of degree : 4<br>
><br>
><br>
> At volume= 301.07 bohr^3<br>
><br>
> C11-C12 is: 0.000062 a.u or 0.915 GPa<br>
><br>
><br>
> Strain energy dE<br>
><br>
> 0.00675692 -1248.135736 0.227374E-12<br>
><br>
> 0.01370014 -1248.135102 0.227374E-12<br>
><br>
> -0.00000019 -1248.135686 0.227374E-12<br>
><br>
> -0.00657965 -1248.135568 0.227374E-12<br>
><br>
> -0.01298805 -1248.135200 0.000000E+00<br>
><br>
><br>
> I changed the lattice constant slightly, and for P~0.1GPa I got almost<br>
> the same result.<br>
><br>
><br>
><br>
> As for MgO which has the same crystal structure with NaCl, I obtained<br>
> almost the same values as the UsersGuide by T.Charpin, 12th July 2001.<br>
><br>
><br>
> Any kind of help will be greatly appreciated.<br>
><br>
><br>
> Best Regards,<br>
><br>
><br>
><br>
> Izumi Hase<br>
><br>
> National Institute of Advanced Industrial Science and Technology (AIST),<br>
> Japan<br>
><br>
><br>
> tel: 029-861-5147 fax: 029-861-5569<br>
><br>
> e-mail: i.hase@aist.go.jp<br>
><br>
><br>
><br>
><br>
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><br>
<br>
--<br>
--------------------------------------------------------------------------<br>
Peter BLAHA, Inst.f. Materials Chemistry, TU Vienna, A-1060 Vienna<br>
Phone: +43-1-58801-165300 FAX: +43-1-58801-165982<br>
Email: blaha@theochem.tuwien.ac.at WIEN2k: <a href="http://www.wien2k.at">http://www.wien2k.at</a><br>
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