# [Pw_forum] ev.x fitting problem

Xiao yxiao.ysu at hotmail.com
Mon Apr 20 18:31:05 CEST 2015

```Dear sir/madam,

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 ev.

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.