## main424

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    // main424.cc is a part of the PYTHIA event generator.
    // Copyright (C) 2025 Torbjorn Sjostrand.
    // PYTHIA is licenced under the GNU GPL v2 or later, see COPYING for details.
    // Please respect the MCnet Guidelines, see GUIDELINES for details.

    // Authors:
    //            Marius Utheim
    //            Leif Lonnblad

    // Keywords:
    //            Hadron‑ion collisions
    //            Optimization

    // The purpose of this example is to generate an initialization file that
    // can be used to speed up initialization in hadron-hadron or hadron-ion runs.
    // By default, it produces data for energies from 10 to 10^6 GeV.
    // All hadron-nucleon and hadron-ion interactions are possible.
    // In other main programs, after initializing with the main424.cmnd file,
    // it is possible to change energy and beam types on a per-event basis.
    // Warning: the generation takes a long time, of the order of 120 minutes,
    // whereof most of it is spent on the nuclear geometry initialization.
    // But, of course, once done you will save time in subsequent runs.

    #include "Pythia8/Pythia.h"

    using namespace Pythia8;

    int main() {

      Pythia pythia;
      // Use Angantyr even when initializing with pp.
      pythia.readString("HeavyIon:mode = 2");

      // Variable energy parameters.
      pythia.readString("Beams:allowVariableEnergy = on");
      pythia.readString("Beams:eCM = 1000000");
      pythia.readString("HeavyIon:varECMMin = 10");
      pythia.readString("HeavyIon:varECMMax = 1000000");
      pythia.readString("HeavyIon:varECMSigFitNPts = 11");

      // Variable beam type parameters.
      pythia.readString("Beams:allowIDASwitch = on");

      // Increased statistics for MPI initialization gives smaller errors.
      //pythia.readString("MultipartonInteractions:nSample = 1000000");

      // Specify where to save. If you set reuseInit = 2, the old files
      // will be replaced/overwritten if they already exist. -1 means that
      // the initialization is stored in the settings, and can be written
      // out.
      pythia.readString("MultipartonInteractions:reuseInit = -1");
      pythia.readString("MultipartonInteractions:initFile = ");
      pythia.readString("HeavyIon:SasdMpiReuseInit = -1");
      pythia.readString("HeavyIon:SasdMpiInitFile = ");
      pythia.readString("HeavyIon:SigFitReuseInit = -1");
      pythia.readString("HeavyIon:SigFitInitFile = ");

      if (!pythia.init()) {
        cout << " Pythia failed to initialize." << endl
             << " Please validate your settings in main424.cmnd." << endl;
        return -1;
      }

      // Note that writeFile will write out all changed settings, not only
      // the ones containing the initialisations, so some manual clean-up
      // is needed to avoid this.
      pythia.readString("HeavyIon:mode = DEFAULT");
      pythia.readString("Next:numberCount = DEFAULT");
      pythia.readString("Next:numberShowEvent = DEFAULT");
      pythia.readString("Next:numberShowInfo = DEFAULT");
      pythia.readString("Next:numberShowLHA = DEFAULT");
      pythia.readString("Next:numberShowProcess = DEFAULT");
      pythia.readString("PartonLevel:all = DEFAULT");
      pythia.readString("ProcessLevel:all = DEFAULT");
      pythia.readString("Tune:ee = DEFAULT");
      pythia.readString("Tune:pp = DEFAULT");
      pythia.settings.writeFile("main424.cmnd");

      // After initializing, we're done.
      return 0;
    }