Fig 2. The thermodynamic cycle for computing the free energy difference between MOP and HMP upon binding to μOR: $\mathrm{\Delta }\mathrm{\Delta }{G}_{\text{bind}}=\mathrm{\Delta }{G}_{\text{bind}}^{\text{MOP}}-\mathrm{\Delta }{G}_{\text{bind}}^{\text{HMP}}=\mathrm{\Delta }{G}_{\text{MOP}\to \text{HMP}}^{\text{bound}}-\mathrm{\Delta }{G}_{\text{MOP}\to \text{HMP}}^{\text{unbound}}.$

The unbound state requires transformation of the ligands alone in solution, since the receptor is the same in both cases.