[Pw_forum] using ev.x

[Tone Kokalj]

Let me just add one more item about bulk modulus.

>From time to time I see the bulk modulus written with too many
significant digits, e.g. 179.3 GPa. Due to the fact that it is a second
derivative calculated numerically and due to a signal/noise that Stefano
mentioned, it is a sensitive quantity. E.g. adding a few additional data
point(s) (or expanding/shrinking the range) may easily change the last
two digits above. 

Here is an example (silver-bulk) of how bulk modulus changes with the
number of data points (the range of scan is from -4% to +4%, and the
fits are performed for several subsets of so-obtained data points):


Bulk Modulus, B0 (in kbar units)::

 ------------+------------+------------+------------+-----------
  # points   | MURNAGHAN  | BIRCH O(1) | BIRCH O(2) |    KEANE  
 ------------+------------+------------+------------+-----------
  15:  1--15 |     896    |     905    |     905    |     610 
  14:  1--14 |     898    |     905    |     904    |     755 
  13:  2--14 |     898    |     905    |     903    |     638 
  12:  2--13 |     900    |     904    |     903    |     748 
  11:  3--13 |     900    |     904    |     904    |     645 
  10:  3--12 |     901    |     904    |     905    |     599 
   9:  4--12 |     901    |     904    |     910    |     665 
   8:  4--11 |     906    |     907    |     909    |     602 
   7:  5--11 |     905    |     907    |     906    |     672 
 ------------+------------+------------+------------+-----------

(the meaning of the first column is "number-of-points: first-point -- last-point")

To add to the Murnaghan vs Keane discussion, it is evident that the
Keane column above displays the largest fluctuations (and also 
different result compared to others). 

Regards, Tone


P.S.: for those interested: the above table was produced by the PWTK 
(http://qe-forge.org/projects/pwtk/) which can calculate lattice
parameter and bulk modulus automatically. For more info see:

Tutorial:
http://pwtk.qe-forge.org/tutorial.html

Examples:
http://pwtk.qe-forge.org/documentation_tclPWTK2F32E20Examples.html

-- 
Anton Kokalj
J. Stefan Institute, Jamova 39, 1000 Ljubljana, Slovenia 
(tel: +386-1-477-3523 // fax:+386-1-477-3822)

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