calculation as UrQMD-2.3. Studies on the thermodynamic properties of UrQMD can be found in [@Bass:1997xw; @Bravina:1998it; @Bravina:2008ra].

The coupling between the UrQMD initial state and the hydrodynamical evolution proceeds when the two Lorentz-contracted nuclei have passed through each other, $t_{\rm start} = {2R}/{\sqrt{\gamma^2
-1}}$ [@Steinheimer:2007iy]. After that, a full (3+1) dimensional ideal hydrodynamic evolution is performed using the SHASTA algorithm [@Rischke:1995ir; @Rischke:1995mt]. The hydrodynamic evolution is stopped, if the energy density $\varepsilon$ of all cells drops below five (default value) times the ground state energy density $\varepsilon_0$ (i.e. $\sim 730 {\rm
MeV/fm}^3$). This criterion corresponds to a T-$\mu_B$-configuration where the phase transition is expected - approximately $T=170$ MeV at $\mu_B=0$. The hydrodynamic fields are then mapped to particle degrees of freedom via the Cooper-Frye equation on an isochronous (in the computational frame) hypersurface. The particle vector information is then transferred back to the UrQMD model, where rescatterings and final decays are calculated using the hadronic cascade. We will further refer to this kind of freeze-out procedure as the isochronous freeze-out (IF). This procedure is explained in detail in [@Petersen:2008dd].

In this paper we introduce another freeze-out procedure to account for the large time dilatation that occurs for fluid elements at 