This section in the config.yaml file contains all general/global configuration options to SMASH.

Available Settings

End_Time (double, required):
The time in fm after which the evolution is stopped. Note that the starting time depends on the chosen Modus.

Randomseed (int, required):
Initial seed for the random number generator. If this is negative, the seed will be randomly generated by the operating system.

Nevents (int, required):
Number of events to calculate.

Use_Grid (bool, optional, default = true):

true - A grid is used to reduce the combinatorics of interaction lookup
false - No grid is used.

Time_Step_Mode (string, optional, default = Fixed):
The mode of time stepping. Possible values:

None - Delta_Time is set to the End_Time. Cannot be used with potentials.
Fixed - Fixed-sized time steps at which collision-finding grid is created. More efficient for systems with many particles. The Delta_Time is provided by user.
For Delta_Time explanation see General.

Metric_Type (string, optional, default = NoExpansion):
Select which kind of expansion the metric should have. This needs only be specified for the sphere modus:

NoExpansion - Default SMASH run, with Minkowski metric
MasslessFRW - FRW expansion going as t^(1/2)
MassiveFRW - FRW expansion going as t^(2/3)
Exponential - FRW expansion going as e^(t/2)

Expansion_Rate (double, optional, default = 0.1):
Corresponds to the speed of expansion of the universe in non minkowski metrics if MetricType is any other than NoExpansion.
It corresponds to \(b_r/l_0\) if the metric type is MasslessFRW or MassiveFRW, and to the parameter b in the Exponential expansion where \(a(t) ~ e^{bt/2}\).

Example: Configuring General Properties

The following example provides a possibility for the General configuration.

General:
    Modus: Collider
    Delta_Time: 0.1
    Testparticles: 1
    Gaussian_Sigma: 1.0
    Gauss_Cutoff_In_Sigma: 3.0
    End_Time: 100.0
    Randomseed: -1
    Nevents: 20
    Use_Grid: True
    Time_Step_Mode: Fixed

In the case of an expanding sphere setup, change the Modus and provide further information about the expansion.

    Modus: Sphere
    MetricType: MasslessFRW
    Expansion_Rate: 0.1

Modus (string, required):
Selects a modus for the calculation, e.g. infinite matter calculation, collision of two particles or collision of nuclei. The modus will be configured in Modi. Recognized values are:

Collider - For collisions of nuclei or compound objects. See smash::ColliderModus
Sphere - For calculations of the expansion of a thermalized sphere. See smash::SphereModus
Box - For infinite matter calculation in a rectangular box. See smash::BoxModus
List - For given external particle list. See smash::ListModus

Delta_Time (double, optional, default: 1.0):
Fixed time step at which the collision-finding grid is recreated, and, if potentials are on, momenta are updated according to the equations of motion. The collision-finding grid finds all the collisions from time t_{beginning_of_timestep} until time t_{beginning_of_timestep} + Delta_Time, and puts them into a vector. The collisions are then sorted in order of occurrence, and particles are propagated from collision to collision. After each performed collision, additional collisions are found for outgoing particles and merged into the sorted vector.

If potentials are on, the Delta_Time should be small enough, typically around 0.1 fm/c. However, if potentials are off, it can be arbitrarily large. In this case it only influences the runtime, but not physics. If Time_Step_Mode = None is chosen, then the user-provided value of Delta_Time is ignored and Delta_Time is set to the End_Time.

Testparticles (int, optional, default = 1):
How many test particles per real particle should be simulated.

Gaussian_Sigma (double, optional, default = 1.0):
Width of gaussians that represent Wigner density of particles, in fm.

Gauss_Cutoff_In_Sigma (double, optional, default = 4.0):
Distance in sigma at which gaussian is considered 0.