Merge pull request #1882 from agiliopadua/docs

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\[ \left< \frac{1}{1 + \exp\left[\left(U_1 - U_0 - \Delta_0^1A \right) /kT \right]} \right>_0 = \left< \frac{1}{1 + \exp\left[\left(U_0 - U_1 + \Delta_0^1A \right) /kT \right]} \right>_1 \]

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\[ \Delta_0^1 A = \int_{\lambda=0}^{\lambda=1} \left( \frac{\partial

    A(\lambda)}{\partial\lambda} \right)_\lambda \mathrm{d}\lambda

\approx \sum_{i=0}^{n-1} w_i \frac{A(\lambda_{i} + \delta) -

  A(\lambda_i)}{\delta} \]

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