# [Pw_forum] About the method to obtain the common tangent for determing the phase transition pressure.

Mon Dec 20 14:07:50 CET 2010

Dear all,

For the first-order phase transitions, the following criteria should be met:
Pressures of the two phases are equal, i.e.,

P_{eq} = - \frac{{d{E_1}}}{{d{V_1}}} = - \frac{{d{E_2}}}{{d{V_2}}}

In other words, the P_{eq} is gradient of common tangent of the two phases E-V
curves obtained from EOS fitting.

This is a mathematical problem, but for the plot of this common tangent, can
someone please give me some hints? What tools/softwares should I use for this