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Nernst's theorem)
The third law of thermodynamics was developed by Walther Nernst and is thus sometimes referred to as Nernst's theorem.
This states that the entropy of a system at zero absolute temperature is a well-defined constant. This is because a system at zero temperature exists in its ground state, so that its entropy is determined only by the degeneracy of the ground state.
A special case of this is systems with a unique ground state, such as crystal lattices. The entropy of these systems as defined by Nernst's theorem is zero (since ln(1) = 0).
See also