See [Takeshi Nishimatsu, Masaya Iwamoto, Yoshiyuki Kawazoe,
and Umesh V. Waghmare: "First-principles accurate total-energy
surfaces for polar structural distortions of Ba2TiO3, PbTiO3,
and SrTiO3: consequences to structural transition temperatures",
Phys. Rev. B, vol.82, p.134106 (2010) doi:10.1103/PhysRevB.82.134106
http://link.aps.org/doi/10.1103/PhysRevB.82.134106 ].

The longer averaging (n_average=200000), the more
beautiful susceptibility plot (strain-susceptibility.gp).

The larger system size, the larger hysteresis in temperature.
