**Nernst's theorem**, sometimes referred to as the

**third law of thermodynamics**, states that the

entropy of a system at

zero absolute temperature is a well-defined constant. This is due to the fact that a system at zero temperature exists in its

ground state, so that its entropy is determined by the

degeneracy of the ground state. Many systems, such as

crystal lattices[?], have a unique ground state, and therefore have zero entropy at at absolute zero (since ln(1) = 0).

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