[Pw_forum] ev.x problem on bulk modulus

[stefano de gironcoli]

Dear Xiao,
    there is a factor of 4 in the volume per unit-cell between sc and 
fcc. just geometry.
    I don't really know what you have calculated but the energy one 
should feed to ev.x is the energy per unit cell as you have used in the 
pw.x calculation. So if in the ev.x input you say that your cell is fcc 
then you should use the energy computed with the fcc unit cell (1 Pd 
atom per cell). If your energy was computed in the sc unit cell (that 
has 4 Pd atoms in the unit cell) then you should specify sc in the ev.x 
input. just be consistent.

stefano

On 20/04/2015 22:31, Xiao wrote:
> Dear Stefano,
> Yes! I used a fcc unit cell. It should have a factor 4. Thank you!!!
> But can you explain why there is a 4 times relation between fcc and sc?
> I don’t really understand why it happens.
>
> best
> xiao
>> On 20 Apr 2015, at 22:09, stefano de gironcoli <degironc at sissa.it 
>> <mailto:degironc at sissa.it>> wrote:
>>
>> Dear Xiao,
>>   are you using the fcc unit cell (1 atom/cell) or the SC one (4 
>> atoms/cell) ?
>>   that would make a factor of 4 in your bulk modulus.
>> stefano
>>
>> On 20/04/2015 21:34, 杨潇 wrote:
>>> Dear All,
>>>
>>> /*Sorry for my mistake of previous email! It should be Ry I used.*/
>>>
>>> As I am using the ev.x to fitting EOS on Pd. I got the wrong bulk 
>>> modulus. Here is my procedure. Please  help me check what’s wrong 
>>> with my fitting?
>>> input data in Å andRy.
>>>
>>> 3.7351 -1.455513098
>>> 3.8129 -1.5041846
>>> 3.8907 -1.527184909
>>> 3.9685 -1.530334715
>>> 4.0463 -1.51819883
>>> 4.1241 -1.4945274
>>>
>>>      Lattice parameter or Volume are in (au, Ang) > Ang
>>> Assuming Angstrom
>>> Enter type of bravais lattice (fcc, bcc, sc, noncubic) > fcc
>>> Enter type of equation of state :
>>> 1=birch1, 2=birch2, 3=keane, 4=murnaghan > 2
>>> then I get the output file like below:
>>>
>>> # equation of state: birch 3rd order.  chisq = 0.3034D-10
>>> # a0 =  7.4514 a.u., k0 = 6726 kbar, dk0 =  5.51 d2k0 = -0.001 emin 
>>> =   -1.53118
>>> # a0 =  3.94309 Ang, k0 = 672.7 GPa,  V0 = 103.430 (a.u.)^3,  V0 = 
>>> 15.327 A^3
>>>
>>> #########################################################################
>>> # Lat.Par E_calc        E_fit       E_diff Pressure      Enthalpy
>>> # Ang Ry           Ry            Ry        GPa         Ry
>>> #########################################################################
>>> 3.73510     -1.45551      -1.45551     0.00000 170.75       -0.43510
>>> 3.81290     -1.50418      -1.50418    -0.00000 89.34       -0.93621
>>> 3.89070     -1.52718      -1.52719     0.00001 30.14       -1.32358
>>> 3.96850     -1.53033      -1.53033    -0.00001 -12.29       -1.61843
>>> 4.04630     -1.51820      -1.51820     0.00001 -42.10       -1.83804
>>> 4.12410     -1.49453      -1.49453    -0.00000 -62.42       -1.99662
>>> ~
>>> The k0 here is 672.7GPa, it should be 180GPa more or less. And I 
>>> fitted same data with another software, it gives me 167GPa.
>>>
>>> So Please Help Me What’s Wrong With My Fitting?
>>>
>>> Thank you!!!
>>>
>>> best
>>> xiao
>>>
>>>
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