The enthalpy of a system is its actual energy (termed the internal energy) plus the product of its volume and the external pressure. Though sometimes termed "heat content,'' the enthalpy in fact includes energy not contained in the system. Enthalpy proves convenient for describing processes in gases and liquids in laboratory environments, if one does not wish to account explicitly for energy stored in the atmosphere by work done when a system expands. It is of little use, however, in describing processes in nanomechanical systems, where work can take many forms: internal energy is then more convenient. Enthalpy is to energy what the Gibbs free energy is to the Helmholtz free energy.

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