Energy must be supplied to remove particles from the surroundings to make space for the creation of the system, assuming that the pressure p remains constant; this is the p V term. The supplied energy must also provide the change in internal energy, U , which includes activation energies, ionization energies, mixing energies, vaporization energies, chemical bond energies, and so forth. Together, these constitute the change in the enthalpy U + p V . For systems at constant pressure, with no external work done other than the p V work, the change in enthalpy is the heat received by the system.