[Wien] Question about ELAST

[Fecher, Gerhard]

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-bounces at zeus.theochem.tuwien.ac.at] im Auftrag von Peter Blaha [pblaha at 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.hase at aist.go.jp
>
>
>
>
> _______________________________________________
> Wien mailing list
> Wien at 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: blaha at 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 at 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