  Recent low-temperature scanning-tunneling microscopy experiments [T. Kumagai
et al., Phys. Rev. B 79, 035423 (2009)] observed the vibrationally induced flip
motion of a hydroxyl dimer (OD)2 on Cu(110). We propose a model to describe
two-level fluctuations and current-voltage characteristics of nanoscale systems
which undergo vibrationally induced switching. The parameters of the model are
based on comprehensive density-functional calculations of the system's
vibrational properties. For the dimer (OD)2 the calculated population of the
high and low conductance states, the I-V, dI/dV, and d2I/dV2 curves are in good
agreement with the experimental results and underlines the different roles
played by the free and shared OD stretch modes of the dimer.
