The default configuration for the Lattice depends on the modus and is in most cases based on some heuristic to approximate the region in space that particles usually reach during the evolution.

Collider

The maximum expected longitudinal velocity is approximated to the speed of light \(v_z=1\) and the maximum expected velocity in each transverse direction is \(v_x=v_y = 0.7\). Assuming an \(R=5\) fm nucleus that is contracted along the z-direction by \(\gamma = \frac{\sqrt{s_{NN}}}{2m_N}\) and the particles propagating until end time, we end up with

\[ z_{\rm max} = \frac{5\,{\rm fm}}{\gamma} + t_{\rm end} \]

\[ x_{\rm max} = y_{\rm max} = 5\,{\rm fm} + 0.7 t_{\rm end}\,.\]

The lattice then covers the range \( -x_{\rm max} < x < x_{\rm max}\), \(-y_{\rm max} < y < y_{\rm max}\) and \(-z_{\rm max} <z< z_{\rm max}\). The cell size in all directions is 0.8 fm. However, if a smearing requiring a lattice (where the smearing length is bound to the lattice cell length) is used, the cell size in z-direction is contracted to \(\frac{0.8\,{\rm fm}}{\gamma}\).

Note: A minimum size of 30 fm is imposed since the heuristic above for determining the lattice expects the end time to be large compared to the nucleus size.

Box and ListBox

The lattice covers exactly the entire box from 0 to box length in x, y and z. The cell size is 0.5 fm and only in this case the lattice is periodic.

Sphere

Since the sphere has an initial Radius, the maximum distance in all directions can be estimated to

\[ x_{\rm max} = y_{\rm max} = z_{\rm max} = R_0 + t_{\rm end} \]

using the speed of light as a maximum expansion velocity. The cell size is 0.8 fm in each direction.

List

There is no default for the List modus. It is basically impossible to foresee how such a modus is used and, hence, the region covered by the lattice has to be actively specified by the user. If an automatic lattice creation is requested, SMASH will terminate with an error.