  Recently, we presented a generalisation of the Jarzynski non-equilibrium work
theorem for phase space mappings. The formalism shows that one can determine
free energy differences from approximate trajectories obtained from molecular
dynamics simulations in which very large timesteps are used. In this work we
test the method by simulating the force induced unfolding of a deca-alanine
helix in vacuum. The excellent agreement between results obtained with a small,
conservative time step of 0.5 fs and results obtained with a time step of 3.2
fs (i.e., close to the stability limit) indicates that the large time step
approach is practical for such complex biomolecules. We further adapt the
method of Hummer and Szabo for the simulation of single-molecule force
spectroscopy experiments to the large time step method. While trajectories
generated with large steps are approximate and may be unphysical - in the
simulations presented here we observe a violation of the equipartition theorem
- the computed free energies are exact in principle. In terms of efficiency,
the optimum time step for the unfolding simulations lies in the range 1-3 fs.
