The equation also plays an important role in the modern theory of phase transitions. It depicts the liquid metals, Mercury and Cesium, quantitatively, and describes most real fluids qualitatively. Consequently it can be regarded as one member of a family of equations of state, that depend on a molecular parameter such as the critical compressibility factor, Z c = p c v c / ( R T c ) , or the Pitzer (acentric) factor, ω ω = − − log ⁡ ⁡ [ p s ( T / T c = 0.7 ) / p c ] − − 1 , where p s / p c = p s ( T / T c , ω ω ) is a dimensionless saturation pressure, and log is the logarithm base 10.