[Wien] Structural phase transition

[Subhasis Panda]

Dear experts,
I've used Wien2k and subsequently Gibbs2 package to study structural phase
transition between two polymorphs of a material. The enthalpy vs pressure
plot at T=0K gives me the transition pressure for a particular transition.
However, when I try to plot difference of Gibbs free energy at P=0 GPa it
doesn't cross zero. But, the literature is showing transition temperature
for the phase transition.
So, my questions are
1) How to determine the transition temperature at P=0 GPa or at any
pressure?
2) How do we get the variation of transition pressure with temperature?
Looking forward for your valuable comments.
Any reference in this direction is most welcomed.
Thank you in advance for the help.
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