Before revision:  The ensemble of proteins attains the equilibrium, \psi_N= \psi_N ^{eq, where the average of \Delta \psi}  _{ N } over fixed mutants is equal to zero.
After revision:  The ensemble of proteins attains the equilibrium, \psi_N= \psi_N ^{eq, where the average of \Delta \psi} _{ ND } over fixed mutants is equal to zero.