  Jarzynski's equality [1] allows us to investigate free energy landscapes
(FELs) by constructing distributions of work performed on a system from an
initial ensemble of states to final states. This work is experimentally
measured by extension-versus-force (EVF) curves. We proposed a new approach
that enables us to reconstruct such FELs without necessity of measuring EVF
curves. We proved that any free energy changes could be computed by measuring
the fluctuations of a harmonic external potential in final states. The main
assumption of our proof is that one should probably treat a potential's minimum
{\lambda} (thought to be control parameter) and time in separate and
independent manners. We recovered Jarzynski's equality from the introduction of
a double Heaviside function. We then applied the approach in molecular dynamics
(MD) simulations to compute the free energy barrier of breaking DNA base pairs
(bps). The free energy barrier for breaking a CG bp in our simulations is
identified as 1.7 +/- 0.2 kcal/mol that is in a very beautiful agreement with
experimental values [2-4] of about 1.9 to 2.1 kcal/mol. Interestingly, the
history of our computed free energies probably represents a favorable
transition pathway.
