  The Jarzynski Equality relates the free energy difference between two
equilibrium states of a system to the average of the work over all irreversible
paths to go from one state to the other. We claim that the derivation of this
equality is flawed, introducing an ad hoc and unjustified weighting factor
which handles improperly the heat exchange with a heat bath. Therefore Liphardt
et al's experiment cannot be viewed as a confirmation of this equality,
although the numerical deviations between the two are small. However, the
Jarzynski Equality may well be a useful approximation, e.g. in measurements on
single molecules in solution.
