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Correlation Time

If the correlation function decays exponentially

\phi (t) \sim \exp{(-t/\tau)}

then the exponential correlation time can be computed as the average

\tau_{\mathrm{exp}} = -\langle \frac{t}{log|\frac{\phi(t)}{\phi(0)}|} \rangle.

If the decay is exponential, then

\int_0^{\infty} dt \phi(t) = \int_0^{\infty} dt \phi(0)\exp{(-t/\tau)} = \tau \phi(0),

which suggests another measure of correlation

\tau_{\mathrm{int}} = \sum_k \frac{\phi(k)}{\phi(0)},

called the integrated correlation time.