  The interaction between edge dislocations and Guinier-Preston zones in an
Al-Cu alloy was analyzed by means of atomistic simulations. The different
thermodynamic functions that determine the features of these obstacles for the
dislocation glide were computed using molecular statics, molecular dynamics and
the nudged elastic band method. It was found that Guinier-Preston zones are
sheared by dislocations and the rate at which dislocations overcome the
precipitate is controlled by the activation energy, $\Delta U$, in agreement
with the postulates of the harmonic transition state theory. Moreover, the
entropic contribution to the Helmholtz activation free energy was in the range
1.3-1.8$k_b$, which can be associated with the typical vibrational entropy of
solids. Finally, an estimation of the initial shear flow stress as a function
of temperature was carried out from the thermodynamic data provided by the
atomistic simulations. Comparison with experimental results showed that the
effect of the random precipitate distribution and of the dislocation character
and dislocation/precipitation orientation has to be taken into account in the
simulations to better reproduce experiments.
