window friction*]{} model [@BRS77; @Fe87]. The collective potential $V(\vec q)$ is defined in our approach as the Helmholtz free energy at given deformation [@PBR96; @BMR96]. The term $F_i(t)$ stands for the random Langevin force which couples collective dynamics to the intrinsic degrees of freedom. We have $F_i(t) \!\!=\!\! \sum_{j} g_{ij}(\vec q) \; G_j(t)$ where the strength tensor $[g(\vec q)]$ is given by the diffusion tensor $[D(\vec q)]$ through $D_{ij} \!\!=\!\! \sum_{k} g_{ik} \; g_{jk}$ and $\vec G(t)$ is a stochastic function. In our model it is assumed that diffusion is related to friction through the Einstein relation $[D(\vec q)] \!=\! [\gamma(\vec q)] \; T$ where T corresponds to the nuclear temperature [@PBR96]. The explicit expressions of these quantities in the TKS parametrization have been presented and discussed in details in [@BMR96].

The friction model we are using is based on a classical concept valid at high energy. When going to lower temperatures this picture can only be considered as an upper limit since nucleon-nucleon collisions become less and less frequent thus reducing friction [@PH91]. We also know that the Einstein relation is in principle only valid at high energy [@HoIv99]. We shall come back to these approximations in section 4.5. 