A reasonable value for τ can be determined from general physical considerations. The largest displacement will be observed for the lightest nuclei, most commonly for hydrogen with the mass of 1 a.m.u. A displacement of more than 20% of the corresponding bond length will probably introduce too much energy into the system, that can even lead to a spurious fragmentation. Therefore, we require σ x to be smaller than about 0.2 Å. For m = 1 a.m.u. this is achieved with τ < 13 fs. From the other side, the momentum distribution σ 2 p = hm 2τ can be thought in terms of temperature, since for MBS σ 2 p,MB = mk B T , i.e. the effective temperature is defined as T eff = h 2kBτ . For velocities of nuclei T eff ≤ 5000 K can be achieved at τ > 0.8 fs. This provide a small range of reasonable values for τ , 1 ≤ τ ≤ 10 fs.