  We propose a method, Temperature Integration, which allows an efficient
calculation of free energy differences between two systems of interest, with
the same degrees of freedom, which may have rough energy landscapes. The method
is based on calculating, for each single system, the difference between the
values of lnZ at two temperatures, using a Parallel Tempering procedure. If our
two systems of interest have the same phase space volume, they have the same
values of lnZ at high-T, and we can obtain the free energy difference between
them, using the two single-system calculations described above. If the phase
space volume of a system is known, our method can be used to calculate its
absolute (versus relative) free energy as well. We apply our method and
demonstrate its efficiency on a toy model of hard rods on a 1-dimensional ring.
