  A fluctuation relation, which is an extended form of the Jarzynski equality,
is introduced and discussed. We show how to apply this relation in order to
evaluate the free energy landscape of simple systems. These systems are
manipulated by varying the external field coupled with a systems' internal
characteristic variable. Two different manipulation protocols are here
considered: in the first case the external field is a linear function of time,
in the second case it is a periodic function of time. While for simple mean
field systems both the linear protocol and the oscillatory protocol provide a
reliable estimate of the free energy landscape, for a simple model
ofhomopolymer the oscillatory protocol turns out to be not reliable for this
purpose. We then discuss the possibility of application of the method here
presented to evaluate the free energy landscape of real systems, and the
practical limitations that one can face in the realization of an experimental
set-up.
