, with X ref being some arbitrarily chosen reference configuration and G (X i ) the absolute free energy corresponding to the i th conformation. m i is the number of equivalent energy wells in the PES of the system compatible with species i (conformers and rotamers), which can be obtained using either a direct counting method or automatic conformational search algorithms. When dealing with non-covalent complexes, this PES degeneracy must also account for the indistinguishability of monomer molecules .