# [Pw_forum] ev.x problem on bulk modulus

Xiao yxiao.ysu at hotmail.com
Mon Apr 20 22:31:43 CEST 2015

```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> 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 and Ry.
>>
>> 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.
>>
>>
>> Thank you!!!
>>
>> best
>> xiao
>>
>>
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