Reply to Comment on 'Failure of the work-Hamiltonian connection for free-energy calculations' by Horowitz and Jarzynski

We show that the Comment [arXiv:0808.1224] by Horowitz and Jarzynski obtains as a main result a general free energy change for a harmonic system that in the macroscopic limit does not recover the textbook expression for the energy change of a Hookean spring. The reason is that Horowitz and Jarzynski improperly identify work with parametric changes of the Hamiltonian instead of with the standard quantity, force times displacement.
Comment: To appear in Physical Review Letters