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
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