[Wien] fitting of elastic constants to energy,strain curves

[Jorissen Kevin]

This is simply a warning to people having trouble with elastic constant calculations in wien2k.
 
 
 
I had problems obtaining sensible (not even good, just sensible) results for elastic constants of cubic TiNi.
 
After some time I discovered that this is due to the fitting routines used to fit the energy vs strain curves obtained for eos, rhomb and tetra strains.
For eos there is no problem.
For rhomb and tetra, the routines are set to use a polynome of maximal degree (ie, number of points - 1).  This leads to absurd results.  (Let me add that I used rather small strains (for which a quadratic fit seems nice enough), which may amplify the pathologies).
The crazy thing is, the available fitting routines are quite sophisticated : they contain criteria to choose the fitting order themselves based on some statistical significance test, but right now this test is overridden!
 
After letting the routines choose the fitting order themselves (with a minimum of order 2), the results got a lot closer to what they should be ...
 
 
 
Secondly : I remember someone was going to generalize the SRC_elast module to some lower symmetry groups.  If this is you : for which symmetries are you doing this?  I would like to calculate elastic constants for rhombohedral systems next; any chance you could help me, or we could help each other?
 
Thanks,
 
 
Kevin Jorissen
 
EMAT - Electron Microscopy for Materials Science   (http://webhost.ua.ac.be/emat/)
Dept. of Physics
 
UA - Universiteit Antwerpen
Groenenborgerlaan 171
B-2020 Antwerpen
Belgium
 
tel  +32 3 2653249
fax + 32 3 2653257
e-mail kevin.jorissen at ua.ac.be
 
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