(color online) The total energy (E)–volume (V) and the pressure (P)–V relations of α, β, and γ phases. The red solid circles are the first-step calculated E. Then we do polynomial fitting to these data and obtain the solid red line. Using the polynomial fitted E–V, we can deduce the P by the relation P = −∂ E/∂V. The integrated values of P, used in the following calculations, are indicated by the black hollow circles.