The existence of an initial conditions subsection in the output section of the configuration file enables the IC output. In addition, all particles that cross the hypersurface of predefined proper time are removed from the evolution. This proper time is taken from the Proper_Time field in the Initial_Conditions subsection when configuring the output. If this information is not provided, the default proper time corresponds to the passing time of the two nuclei, where all primary interactions are expected to have occured:

\[ \tau_0 = (r_\mathrm{p} \ + \ r_\mathrm{t}) \ \left(\left(\frac{\sqrt{s_\mathrm{NN}}} {2 \ m_\mathrm{N}}\right)^2 - 1\right)^{-1/2} \]

Therein, \( r_\mathrm{p} \) and \( r_\mathrm{t} \) denote the radii of the projectile and target nucleus, respectively, \( \sqrt{s_\mathrm{NN}}\) is the collision energy per nucleon and \( m_\mathrm{N} \) the nucleon mass. Note though that, if the passing time is smaller than 0.5 fm, the default proper time of the hypersurface is taken to be \(\tau = 0.5 \) as a minimum bound to ensure the proper time is large enough to also extract reasonable initial conditions at RHIC/LHC energies. If desired, this lowest possible value can also be specified in the configuration file in the Lower_Bound field.
Once initial conditions are enabled, the output file named SMASH_IC (followed by the appropriate suffix) is generated when SMASH is executed.
The output is available in Oscar1999, Oscar2013, binary and ROOT format, as well as in an additional ASCII format (see Initial conditions). The latter is meant to directly serve as an input for the vHLLE hydrodynamics code (I. Karpenko, P. Huovinen, M. Bleicher: Comput. Phys. Commun. 185, 3016 (2014)).

Oscar output

In case of the Oscar1999 and Oscar2013 format, the structure is identical to the Oscar Particles Format (see OSCAR particles format).
In contrast to the usual particles output however, the initial conditions output provides a list of all particles removed from the evolution at the time when crossing the hypersurface. This implies that neither the initial particle list nor the particle list at each time step is printed.
The general Oscar structure as described in OSCAR particles format is preserved.

Binary output

The binary initial conditions output also provides a list of all particles removed from the evolution at the time when crossing the hypersurface. For each removed particle a 'p' block is created stores the particle data. The general binary output structure as described in Binary format is preserved.

ROOT output

The initial conditions output in shape of a list of all particles removed from the SMASH evolution when crossing the hypersurface is also available in ROOT format. Neither the initial nor the final particle lists are printed, but the general structure for particle TTrees, as described in ROOT format, is preserved.

The ASCII initial conditions output (SMASH_IC.dat) contains a list of particles on a hypersurface of constant proper time. This output is formatted such that it is directly compatible with the vHLLE hydrodynamics code (I. Karpenko, P. Huovinen, M. Bleicher: Comput. Phys. Commun. 185, 3016 (2014)). As a consequence, spectators are not written to the ASCII IC output as they would need to be excluded anyways in order to initialize the hydrodynamics evolution. Note though that for all other output formats the full particle list is printed to the IC output, including spectators. The particle data is provided in the computational frame. For further details, see Initial conditions.

The ASCII initial conditions output is formatted as follows:
Header

# **smash_version** initial conditions: hypersurface of constant proper time
# tau x y eta mt px py Rap pdg charge baryon_number strangeness
# fm fm fm none GeV GeV GeV none none e none none

The header consists of 3 lines starting with a '#', containing the following information:

SMASH-version and the information, that 'initial conditions' are provided
The header with all column names
The units of all column quantities

Output block header

The ASCII initial conditions output is, similar to the OSCAR output, based on a block structure, where each block consists of 1 event. The header for a new event is structured as follows:

# event ev_num start

where

ev_num: The number of the current event

Note that 'event' and 'start' are no variables, but words that are printed in the header.

Particle line

The particle lines are formatted as follows:

tau x y eta mt px py Rap pdg charge baryon_number strangeness

smash::pdg::eta

constexpr int eta

η.

Definition: pdgcode_constants.h:83

where

tau: Proper time of the particle
x, y: Cartesian x and y coordinates of the particle
eta: Space-time rapidity of the particle
mt: Transverse mass of the particle
px, py: x and y components of the particle's momentum
Rap: Momentum space rapidity of the particle
pdg: PDG code of the particle (see http://pdg.lbl.gov/). It contains all quantum numbers and uniquely identifies its type. every particle in the event.
charge: electric charge of the particle
baryon_number: baryon number of the particle
strangeness: strangeness of the particle

Event end line

The end of an event is indicated by the following line:

# event ev_num end

where

ev_num: The number of the current event

Note that 'event' and 'end' are no variables, but words that are printed in the header.

Note: If SMASH is run with test particles (necessary e.g. for potentials), the ASCII output will contain Ntest * Npart particle entries. Remember to weigh each of those particles with 1/Ntest.