[Pw_forum] why total energy vs. lattice parameter is not a parabola?

[vu ongphuong]

Dear all users,
   
  I calculate the lattice canstant of SrTiO3. When I plot the  total energy vs. lattice parameter, the shape is not a parabola even the  lattice parameter is taken in very near-minimum-region. The the lattice canstant obtained is 3.851Angstrong (the  experimental value 3.905)
   
  When I calculate with VASP package the result is 3.86 and the total energy vs. lattice parameter is really a parabola.
   
  Anybody can explain me?

 		
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