[Wien] Question about ELAST

[Peter Blaha]

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
>
>
>
>
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-- 
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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
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