Hi, I have been trying to understand what is enthalpy (H = U + PV). What has been my current understanding is it is the energy content in a system. And this nicely brings together the energy contents in terms of the 3 state variables, Temperature, Pressure and Volume. If enthalpy is the energy content of the system. ie, the energy required to create the system given its Temperature, Volume and Pressure. Why is energy contained in the mass of the system (E= mc^2) not considered here ? Shouldn't it be H = U + PV + mc^2 ?