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Enthalpy is a measure of energy in a thermodynamic system:

It includes the internal energy, which is the energy required to create > a system, and the amount of energy required to make room for it by displacing its environment and establishing its volume and pressure.

Source: Wikipedia / Enthalpy

My question is related to internal energy not enthalpy.

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    $\begingroup$ That depends on your precise definition of internal energy and what you mean by "exist", but e.g. an ideal gas at zero temperature would probably have zero internal energy. $\endgroup$ – ACuriousMind♦ Feb 3 '16 at 16:31
  • $\begingroup$ It depends on what you mean by "system". One could argue that a mechanical system such as a point mass, or a solid, has zero internal energy as it has no microscopic internal structure. All its energy is in the form of marcroscopic mechanical energy. $\endgroup$ – Dimitri Feb 3 '16 at 16:33
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    $\begingroup$ null internal energy is just a reference. In most thermodynamics systems ( try to account of the energy vacuum ) , it is impossible to calculate the total internal energy ( wiki ). $\endgroup$ – user46925 Feb 3 '16 at 16:36
  • $\begingroup$ Complete vaccum has no internal energy. But still, you may consider it as a system which can have coordinates.Doesn't complete vaccum exist!! $\endgroup$ – Anubhav Goel Feb 3 '16 at 16:50
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    $\begingroup$ Also, remember that only $\mathrm dU$ is defined: $U$ is always underdetermined (you can shift $U\to U+\text{const}$ for any constant, without changing the physics). $\endgroup$ – AccidentalFourierTransform Feb 3 '16 at 19:00

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