Ansys Employee

Couple of points.

The difference in the formation enthalpy is the latent heat: you add heat to vapourise so get the sign right.  So, water-liquid can be set as zero and vapour then has a value or vice versa. The formation enthalpy in the materials panel also accounts for the chemical energy incase you want to burn the phase too. 

What's the specific heat capacity of gas relative to water?