where Δ Δ U denotes the difference between the internal energy of the given state and that of the reference state, and the E i are the various energies transferred to the system in the steps from the reference state to the given state. It is the energy needed to create the given state of the system from the reference state. From a non-relativistic microscopic point of view, it may be divided into microscopic potential energy, U micro,pot , and microscopic kinetic energy, U micro,kin , components: