[Wien] optimization for 225 (cubic) space group

[fatima DFT]

Dear All,
I finished an optimisation for 225 space group with -15% to 25% volume
change.

If I plot the optimization cure for different vol parameters, I see the
shape of ENE vs. Vol curve changes and the optimised lattice parametes also
changes (at third digit).

If I plot the curve for -15 to 25% then I get the below curve
*https://sites.google.com/site/dftfatima5/letter/mailing-list-queries/
<https://sites.google.com/site/dftfatima5/letter/mailing-list-queries/>*1.ps
<https://sites.google.com/site/dftfatima5/letter/mailing-list-queries?pli=1/1.ps>

<https://sites.google.com/site/dftfatima5/letter/mailing-list-queries?pli=1/1.ps>

and the optimised lattice parameters are

21.1924 bohr =    11.2145 Ang

For -10 to 25%,
<https://sites.google.com/site/dftfatima5/letter/mailing-list-queries?pli=1/2.ps>
https://sites.google.com/site/dftfatima5/letter/mailing-list-queries/2.ps
<https://sites.google.com/site/dftfatima5/letter/mailing-list-queries?pli=1/1.ps>

21.1984 bohr =    11.2177 Ang

For -5 to 25%
<https://sites.google.com/site/dftfatima5/letter/mailing-list-queries?pli=1/3.ps>
https://sites.google.com/site/dftfatima5/letter/mailing-list-queries/3.ps
<https://sites.google.com/site/dftfatima5/letter/mailing-list-queries?pli=1/1.ps>

21.1915 bohr =    11.2140 Ang

0.0 to 25 %
<https://sites.google.com/site/dftfatima5/letter/mailing-list-queries?pli=1/4.ps>
https://sites.google.com/site/dftfatima5/letter/mailing-list-queries/4.ps
<https://sites.google.com/site/dftfatima5/letter/mailing-list-queries?pli=1/1.ps>

21.1917 bohr =    11.2141 Ang
 for 5.0  to 25 %
https://sites.google.com/site/dftfatima5/letter/mailing-list-queries/5.ps
<https://sites.google.com/site/dftfatima5/letter/mailing-list-queries?pli=1/1.ps>
21.2026 bohr =    11.2199 Ang


My query is:

1. Why the optimised lattice parameters are changing if we select different
set of data points? The ground state should be same in all cases. Is it?
2.  We see from the lattice parameter the change is at third order, can we
take any lattice parameters (a0)? How much the third digit changes the
electronic properties?


Warm regards
Fatima
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