where the a subscript outside the angle brackets denotes an ensemble average with the solute-solvent b interactions turned off (i.e. λ a = 1 and λ b = 0), and the 1 subscript denotes the average with respect to full solute-solvent coupling (i.e. λ a = 1 and λ b = 1). The first expression in Eq. 20 shows that the total en-