where W s 0 and W f 0 are defined analogously to W 0 in Eq. ( ) but with Rs 0 or Rf 0 replacing R0 . Equation ( ) for E k is a natural generalization of Eq. ( ) for E 0 , and corresponds to solving the Schrödinger equation in the fixed reaction field of the equilibrated ground state. The first nonequilibrium correction arises in first-order perturbation theory and is given by