To calculate heats of formation, some care is needed, for it usually requires a thermodynamic cycle (Figure ) and the mixed-use of values from both experiments and QM calculations that usually have different zero of energy references. For a given molecule, "M ", the heat of formation at 298 K, ∆H f (M, 298 K), is defined as the difference in enthalpy between the molecule and its constituent elements in their standard state. Using CH 4 as an example: