Box

Initial_Condition (string, required, no default):
Controls initial momentum distribution of particles.

• "peaked momenta" - All particles have momentum \(p = 3 \cdot T\), where T is the temperature. Directions of momenta are uniformly distributed.
• "thermal momenta" - Momenta are sampled from a Maxwell-Boltzmann distribution.

Length (double, required):
Length of the cube's edge, in fm.

Temperature (double, required):
Temperature in the box, in GeV.

Start_Time (double, required):
Starting time of the simulation. All particles in the box are initialized with \(x^0\) = Start_Time.

Init_Multiplicities (int, required):
Map of PDG number and quantity of this PDG number. Controls how many particles of each sort will be initialized.

Use_Thermal_Multiplicities (bool, optional, default = false):
If this option is set to true then Init_Multiplicities are ignored and the box is initialized with all particle species of the particle table that belong to the hadron gas equation of state, see HadronGasEos::is_eos_particle(). The multiplicities are sampled from Poisson distributions \( Poi(n_i V) \), where \( n_i \) are the grand-canonical thermal densities of the corresponding species and \( V \) is the box volume. This option simulates the grand-canonical ensemble, where the number of particles is not fixed from event to event.

Baryon_Chemical_Potential (double, optional, default = 0.0):
Baryon chemical potential \( \mu_B \) used in case if Use_Thermal_Multiplicities is true to compute thermal densities \( n_i \).

Strange_Chemical_Potential (double, optional, default = 0.0):
Strangeness chemical potential \( \mu_S \) used in case if Use_Thermal_Multiplicities is true to compute thermal densities \( n_i \).

Examples: Configuring a Box Simulation

The following example configures an infinite matter simulation in a Box with 10 fm cube length at a temperature of 200 MeV. The particles are initialized with thermal momenta at a start time of 10 fm. The particle numbers at initialization are 100 \( \pi^+ \), 100 \( \pi^0 \), 100 \( \pi^- \), 50 protons and 50 neutrons.

Modi:
    Box:
        Length: 10.0
        Temperature: 0.2
        Initial_Condition: "thermal momenta"
        Start_Time: 10.0
        Init_Multiplicities:
            211: 100
            111: 100
            -211: 100
            2212: 50
            2112: 50

On the contrary, it is also possible to initialize a thermal box based on thermal multiplicities. This is done via

Modi:
    Box:
        Length: 10.0
        Temperature: 0.2
        Use_Thermal_Multiplicities: True
        Baryon_Chemical_Potential: 0.0
        Strange_Chemical_Potential: 0.0

Note

The box modus is most useful for infinite matter simulations with thermal and chemical equilibration and detailed balance. Detailed balance can however not be conserved if 3-body decays (or higher) are performed. To yield useful results applying a SMASH box simulation, it is therefore necessary to modify the provided default particles.txt and decaymodes.txt by removing 3-body and higher order decays from the decaymodes file and all corresponding particles that can no longer be produced from the particles file. In addtion, strings need to be turned off, since they also break detailed balance due to lacking backreactions.
SMASH is shipped with example files (config.yaml, particles.txt, decaymodes.txt) meeting the above mentioned requirements to set up an infinite matter simulation. They are located in /input/box. To run SMASH with the provided example files, execute

   ./smash -i INPUT_DIR/box/config.yaml -p INPUT_DIR/box/particles.txt -d
INPUT_DIR/box/decaymodes.txt 

Where 'INPUT_DIR' needs to be replaced by the path to the input directory ('../input', if the build directory is located in the smash folder).