{\displaystyle \Delta F(\mathbf {A} \to \mathbf {B} )=F_{\mathbf {B} }-F_{\mathbf {A} }=-k_{\text{B}}T\ln \left\langle \exp \left(-{\frac {E_{\text{B}}-E_{\mathbf {A} }}{k_{\text{B}}T}}\right)\right\rangle _{\mathbf {A} },}