  Mechanical unfolding and refolding of ubiquitin are studied by Monte Carlo
simulations of a Go model with binary variables. The exponential dependence of
the time constants on the force is verified, and folding and unfolding lengths
are computed, with good agreement with experimental results. Furthermore, the
model exhibits intermediate kinetic states, as observed in experiments.
Unfolding and refolding pathways and intermediate states, obtained by tracing
single secondary structure elements, are consistent with simulations of
previous all-atom models and with the experimentally observed step sizes.
