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Statistical Mechanics · graduate

Fluctuation-Response Relation

Equilibrium fluctuations determine linear response coefficients.

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Fluctuation-Response Relation

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Heat Capacity

In the canonical ensemble, energy fluctuations determine the heat capacity:

CV=E2E2kBT2.C_V=\frac{\langle E^2\rangle-\langle E\rangle^2}{k_BT^2}.

Susceptibility

More generally, the response of an observable to a conjugate field is related to the equilibrium variance of that observable. This is a central bridge between microscopic fluctuations and macroscopic measurements.

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