Pythia8_i.cxx

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/*
  Copyright (C) 2002-2025 CERN for the benefit of the ATLAS collaboration
*/
#include "Pythia8_i/Pythia8_i.h"
#include "Pythia8_i/UserProcessFactory.h"
#include "Pythia8_i/UserResonanceFactory.h"
#include "PathResolver/PathResolver.h"
 
#include "GeneratorObjects/McEventCollection.h"
#include <ranges>
#include <charconv>
 
// calls to fortran routines
#include "AthenaKernel/RNGWrapper.h"
#include "TruthUtils/MagicNumbers.h"
 
#include <sstream>
// For limits
#include <limits>
 
 
namespace {
  // Name of random number stream
  std::string s_pythia_stream{"PYTHIA8_INIT"};
}
 
 
// fix Pythia8 shower weights change in conventions
#define PYTHIA8_NWEIGHTS nWeights
#define PYTHIA8_WEIGHT weight
#define PYTHIA8_WLABEL weightLabel
#define PYTHIA8_CONVERSION 1.0
 
#ifdef PYTHIA_VERSION_INTEGER
  #undef PYTHIA8_NWEIGHTS
  #undef PYTHIA8_WEIGHT
  #undef PYTHIA8_WLABEL
  #if PYTHIA_VERSION_INTEGER > 8303
    #define PYTHIA8_NWEIGHTS nWeightGroups
  #else
    #define PYTHIA8_NWEIGHTS nVariationGroups
  #endif
  #define PYTHIA8_WEIGHT getGroupWeight
  #define PYTHIA8_WLABEL getGroupName
#endif

 
namespace {
 
std::string py8version()
{
  static const std::string incdir (PY8INCLUDE_DIR);
  std::string::size_type pos = incdir.find ("/pythia8/");
  if (pos == std::string::npos) return "";
  pos += 9;
  std::string::size_type pos2 = incdir.find ("/", pos);
  if (pos2 == std::string::npos) pos2 = incdir.size();
  return incdir.substr (pos, pos2-pos);
}
 
 
}

Pythia8_i::Pythia8_i(const std::string &name, ISvcLocator *pSvcLocator)
: GenModule(name, pSvcLocator)
{
  m_particleIDs["PROTON"]      = PROTON;
  m_particleIDs["ANTIPROTON"]  = ANTIPROTON;
  m_particleIDs["ELECTRON"]    = ELECTRON;
  m_particleIDs["POSITRON"]    = POSITRON;
  m_particleIDs["NEUTRON"]     = NEUTRON;
  m_particleIDs["ANTINEUTRON"] = ANTINEUTRON;
  m_particleIDs["MUON"]        = MUON;
  m_particleIDs["ANTIMUON"]    = ANTIMUON;
  m_particleIDs["LEAD"]        = LEAD;
  m_particleIDs["OXYGEN"]      = OXYGEN;
  m_particleIDs["HELIUM"]      = HELIUM;
 
}

Pythia8_i::~Pythia8_i() {
 
  #ifndef PYTHIA8_3SERIES
//  if(m_userHookPtr != 0) delete m_userHookPtr;
 
//  for(UserHooksPtrType ptr: m_userHooksPtrs){
//    delete ptr;
//  }
  #endif
}

StatusCode Pythia8_i::genInitialize() {
 
  ATH_MSG_DEBUG("Pythia8_i from genInitialize()");
 
  ATH_MSG_INFO("XML Path is " + xmlpath());
  m_pythia = std::make_unique<Pythia8::Pythia> (xmlpath());
  m_runinfo = std::make_shared<HepMC3::GenRunInfo>();
  struct HepMC3::GenRunInfo::ToolInfo generator={std::string("Pythia8"),py8version(),std::string("Used generator")};
  m_runinfo->tools().push_back(std::move(generator));   
 
  bool canInit = true;
 
  m_version = m_pythia->settings.parm("Pythia:versionNumber");
 
  s_pythia_stream = "PYTHIA8_INIT";
 
  // By default add "nominal" to the list of shower weight names
  m_showerWeightNames = m_showerWeightNamesProp.value();
  m_showerWeightNames.insert(m_showerWeightNames.begin(), "nominal");
 
  // We do explicitly set tune 4C, since it is the starting point for many other tunes
  // Tune 4C for pp collisions
  m_pythia->readString("Tune:pp = 5");
 
  // also use CTEQ6L1 from LHAPDF 6 by default
  // can be over-written using JO
  m_pythia->readString("PDF:pSet= LHAPDF6:cteq6l1");
 
  // switch off verbose event print out
  m_pythia->readString("Next:numberShowEvent = 0");
 
  // Add flag to switch off from JO the Pythia8ToHepMC::print_inconsistency internal variable
  m_pythia->settings.addFlag("AthenaPythia8ToHepMC:print_inconsistency",true);
  
  // Revert the recoil strategy to the old default of 1, ie. 'recoil to color'
  // In 8.314, the default option was changed to 0, 'recoil to top' and we 
  // revert it back unless 'recoil to top' 
  // is explicitly needed for the samples 
  if (m_version > 8.313) m_pythia->readString("TimeShower:recoilStrategyRF = 1");
 
  // Add UserHooks first because these potentially add new settings that must exist prior to parsing commands
 
  for(const auto &hook: m_userHooks){
    ATH_MSG_INFO("Adding user hook " + hook + ".");
    bool canSetHook = true;
    if (hook == "SuppressSmallPT") {
        m_SuppressSmallPT = new Pythia8::SuppressSmallPT(m_pt0timesMPI,m_numberAlphaS,m_sameAlphaSAsMPI);
        canSetHook=m_pythia->setUserHooksPtr(PYTHIA8_PTRWRAP(m_SuppressSmallPT));
}
    else {
      m_userHooksPtrs.push_back(PYTHIA8_PTRWRAP(Pythia8_UserHooks::UserHooksFactory::create(hook)) );
      canSetHook = m_pythia->addUserHooksPtr(m_userHooksPtrs.back());
}
    if(!canSetHook){
      ATH_MSG_ERROR("Unable to set requested user hook.");
      ATH_MSG_ERROR("Pythia 8 initialisation will FAIL!");
      canInit=false;
    }
  }
 
  for(const std::pair<const std::string, double> &param : Pythia8_UserHooks::UserHooksFactory::userSettings<double>()){
    m_pythia->settings.addParm(param.first, param.second, false, false, 0., 0.);
  }
 
  for(const std::pair<const std::string, int> &param : Pythia8_UserHooks::UserHooksFactory::userSettings<int>()){
    m_pythia->settings.addMode(param.first, param.second, false, false, 0., 0.);
  }
 
  for(const std::pair<const std::string, bool> &param : Pythia8_UserHooks::UserHooksFactory::userSettings<bool>()){
    m_pythia->settings.addFlag(param.first, param.second);
  }
 
  for(const std::pair<const std::string, std::string> &param : Pythia8_UserHooks::UserHooksFactory::userSettings<std::string>()){
    m_pythia->settings.addWord(param.first, param.second);
  }
 
  if( ! m_athenaTool.empty() ){
    if(m_athenaTool.retrieve().isFailure()){
      ATH_MSG_ERROR("Unable to retrieve Athena Tool for custom Pythia processing");
      return StatusCode::FAILURE;
    }
    else {
      StatusCode status = m_athenaTool->InitializePythiaInfo(*m_pythia);
      if(status != StatusCode::SUCCESS) return status;
    }
  }
 
  // Now apply the settings from the JO
  for(const std::string &cmd : m_commands){
 
    if(cmd.compare("")==0) continue;
    else if (cmd.find("Beams:id") != std::string::npos ) {
      ATH_MSG_ERROR("With command '" << cmd << "' you are trying to set a beam different from p: please use the Beam1/Beam2 properties instead:"); 
      ATH_MSG_ERROR("                               example:  genSeq.Pythia8.Beam1 = 'LEAD'");
      ATH_MSG_ERROR("                                         genSeq.Pythia8.Beam2 = 'ANTINEUTRON'");
      return StatusCode::FAILURE;
    }
    else if (cmd.find("Beams:frameType") != std::string::npos ) {
      m_override_transform_beamenergy = true;
      ATH_MSG_WARNING(" Found an explicit 'Beams:frameType' command: this will override transform beams momenta/energy parameters, regardless of its requested value. ");
    }
    
    bool read = m_pythia->readString(cmd);
 
    if(!read){
      ATH_MSG_ERROR("Pythia could not understand the command '"<< cmd<<"'");
      return StatusCode::FAILURE;
    }
  }
 
  PDGID beam1 = m_particleIDs[m_beam1];
  PDGID beam2 = m_particleIDs[m_beam2];
 
  ATH_MSG_INFO("Beam1 = "<<beam1);
  ATH_MSG_INFO("Beam2 = "<<beam2);
 
  if(beam1 == 0 || beam2 == 0){
    ATH_MSG_ERROR("Invalid beam particle!");
    return StatusCode::FAILURE;
  }
 
 
 
  if(m_useRndmGenSvc){
 
    ATH_MSG_INFO(" !!!!!!!!!!!!  WARNING ON PYTHIA RANDOM NUMBERS !!!!!!!!!! ");
    ATH_MSG_INFO("           THE ATHENA SERVICE AthRNGenSvc IS USED.");
    ATH_MSG_INFO(" !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! ");
 
    m_atlasRndmEngine = std::make_shared<customRndm>();
    CLHEP::HepRandomEngine* rndmEngine = getRandomEngineDuringInitialize(s_pythia_stream, m_randomSeed, m_dsid); // NOT THREAD-SAFE
    m_atlasRndmEngine->init(rndmEngine);
#if PYTHIA_VERSION_INTEGER >= 8310
    m_pythia->setRndmEnginePtr(m_atlasRndmEngine);
#else
    m_pythia->setRndmEnginePtr(m_atlasRndmEngine.get());
#endif
    s_pythia_stream = "PYTHIA8";
  }else{
    ATH_MSG_INFO(" !!!!!!!!!!!!  WARNING ON PYTHIA RANDOM NUMBERS !!!!!!!!!! ");
    ATH_MSG_INFO("    THE STANDARD PYTHIA8 RANDOM NUMBER SERVICE IS USED.");
    ATH_MSG_INFO("    THE ATHENA SERVICE AthRNGSvc IS ***NOT*** USED.");
    ATH_MSG_INFO(" !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! ");
  }
 
  if(m_userProcess != ""){
    m_procPtr = Pythia8_UserProcess::UserProcessFactory::create(m_userProcess);
  }
 
 
 
  if(m_userResonances.value() != ""){
 
    std::vector<std::string> resonanceArgs;
 
    for (auto&& part : std::views::split(m_userResonances.value(), ':')) resonanceArgs.emplace_back(part.begin(), part.end());
    if(resonanceArgs.size() != 2){
      ATH_MSG_ERROR("Cannot Understand UserResonance job option!");
      ATH_MSG_ERROR("You should specify it as a 'name:id1,id2,id3...'");
      ATH_MSG_ERROR("Where name is the name of your UserResonance, and id1,id2,id3 are a comma separated list of PDG IDs to which it is applied");
      canInit = false;
    }
    std::vector<std::string> resonanceIds;
    for (auto&& part : std::views::split(resonanceArgs.back(), ',')) resonanceIds.emplace_back(part.begin(), part.end());
    if(resonanceIds.size()==0){
      ATH_MSG_ERROR("You did not specifiy any PDG ids to which your user resonance width should be applied!");
      ATH_MSG_ERROR("You should specify a list as 'name:id1,id2,id3...'");
      ATH_MSG_ERROR("Where name is the name of your UserResonance, and id1,id2,id3 are a comma separated list of PDG IDs to which it is applied");
      canInit=false;
    }
 
    for(std::vector<std::string>::const_iterator sId = resonanceIds.begin();
        sId != resonanceIds.end(); ++sId){
      int idResIn = 0;
      auto result = std::from_chars(sId->data(), sId->data() + sId->size(), idResIn);
      if (result.ec != std::errc()) {
          ATH_MSG_ERROR("Invalid resonance ID: " + *sId);
      }
      m_userResonancePtrs.push_back(Pythia8_UserResonance::UserResonanceFactory::create(resonanceArgs.front(), idResIn));
    }
 
    for(std::shared_ptr<Pythia8::ResonanceWidths>& resonance : m_userResonancePtrs) {
#if PYTHIA_VERSION_INTEGER >= 8310
      m_pythia->setResonancePtr(resonance);
#else
      m_pythia->setResonancePtr(resonance.get());
#endif
    }
 
  }
 
  if(m_particleDataFile != "") {
    if(!m_pythia->particleData.reInit(m_particleDataFile, true)){
      ATH_MSG_ERROR("Unable to read requested particle data table: " + m_particleDataFile + " !!");
      ATH_MSG_ERROR("Pythia 8 initialisation will FAIL!");
      canInit = false;
    }
  }
 
  if(m_lheFile != ""){
    if(m_procPtr){
      ATH_MSG_ERROR("Both LHE file and user process have been specified");
      ATH_MSG_ERROR("LHE input does not make sense with a user process!");
      canInit = false;
    }
 
    canInit = canInit && m_pythia->readString("Beams:frameType = 4");
    canInit = canInit && m_pythia->readString("Beams:LHEF = " + m_lheFile);
    if(!canInit){
      ATH_MSG_ERROR("Unable to read requested LHE file: " + m_lheFile + " !");
      ATH_MSG_ERROR("Pythia 8 initialisation will FAIL!");
    }
  }else{
    if (!m_override_transform_beamenergy){
      canInit = canInit && m_pythia->readString("Beams:frameType = 1");
      canInit = canInit && m_pythia->readString("Beams:eCM = " + std::to_string(m_collisionEnergy));
    }
    canInit = canInit && m_pythia->readString("Beams:idA = " + std::to_string(beam1));
    canInit = canInit && m_pythia->readString("Beams:idB = " + std::to_string(beam2));
  }
 
  if(m_procPtr){
#if PYTHIA_VERSION_INTEGER >= 8310
    if(!m_pythia->setSigmaPtr(m_procPtr))
#else
    if(!m_pythia->setSigmaPtr(m_procPtr.get()))
#endif
    {
      ATH_MSG_ERROR("Unable to set requested user process: " + m_userProcess + " !!");
      ATH_MSG_ERROR("Pythia 8 initialisation will FAIL!");
      canInit = false;
    }
  }
 
  StatusCode returnCode = StatusCode::SUCCESS;
 
  m_pythia->particleData.listXML(m_outputParticleDataFile.value().substr(0,m_outputParticleDataFile.value().find("xml"))+"orig.xml");
  m_pythia->settings.writeFile("Settings_before.log",true);
 
  if(canInit){
    canInit = m_pythia->init();
  }
 
  if(!canInit){
    returnCode = StatusCode::FAILURE;
    ATH_MSG_ERROR(" *** Unable to initialise Pythia !! ***");
  }
 
  m_pythia->particleData.listXML(m_outputParticleDataFile);
  m_pythia->settings.writeFile("Settings_after.log",true);
 
  //counter for event failures;
  m_failureCount = 0;
 
  m_internal_event_number = 0;
 
  // Set set_print_inconsistency to Athena corresponding flag (allowing to change it from JO)
  m_pythiaToHepMC.set_print_inconsistency(  m_pythia->settings.flag("AthenaPythia8ToHepMC:print_inconsistency")  );
 
  m_pythiaToHepMC.set_store_pdf(true);
  m_pythiaToHepMC.set_store_weights(false);
 
  return returnCode;
}

StatusCode Pythia8_i::callGenerator(){
 
  ATH_MSG_DEBUG(">>> Pythia8_i from callGenerator");
  if(m_useRndmGenSvc && m_useReseed){
    //Re-seed the random number stream
    long seeds[7];
    const EventContext& ctx = Gaudi::Hive::currentContext();
    ATHRNG::calculateSeedsMC21(seeds, s_pythia_stream,  ctx.eventID().event_number(), m_dsid, m_randomSeed);
    m_atlasRndmEngine->getEngine()->setSeeds(seeds, 0); // NOT THREAD-SAFE
    m_seeds.clear();
    m_seeds.push_back(seeds[0]);
    m_seeds.push_back(seeds[1]);
  }
 
  bool status = m_pythia->next();
 
  StatusCode returnCode = StatusCode::SUCCESS;
 
  if(!status){
    ++m_failureCount;
    if(m_failureCount >= m_maxFailures){
      ATH_MSG_ERROR("Exceeded the max number of consecutive event failures.");
      returnCode =  StatusCode::FAILURE;
    }else{
      ATH_MSG_INFO("Event generation failed - re-trying.");
      returnCode = this->callGenerator();
    }
  }
 
  ATH_MSG_DEBUG("Now checking with Tool.empty() second time");
  if( ! m_athenaTool.empty() ){
      StatusCode stat = m_athenaTool->ModifyPythiaEvent(*m_pythia);
      if(stat != StatusCode::SUCCESS) returnCode = stat;
  }
 
  m_failureCount = 0;
 
  m_nAccepted += 1.;
  // some CKKWL merged events have zero weight (or unfilled event).
  // start again with such events
 
  double eventWeight = m_pythia->info.mergingWeight()*m_pythia->info.weight();
 
 
  if(returnCode != StatusCode::FAILURE &&
     // Here this is a double, but it may be converted into float at some point downstream
     (std::abs(eventWeight) < std::numeric_limits<float>::min() ||
      m_pythia->event.size() < 2)){
 
       returnCode = this->callGenerator();
     } else if ( std::abs(eventWeight) < std::numeric_limits<float>::min() &&
                 std::abs(eventWeight) > std::numeric_limits<double>::min() ){
       ATH_MSG_WARNING("Found event weight " << eventWeight << " between the float and double precision limits. Rejecting event.");
     } else {
       m_nMerged += eventWeight;
       ++m_internal_event_number;
 
       // For FxFx cross section:
       m_sigmaTotal+=m_pythia->info.weight();
     }
 
  return returnCode;
}

StatusCode Pythia8_i::fillEvt(HepMC::GenEvent *evt){
 
  ATH_MSG_DEBUG(">>> Pythia8_i from fillEvt");
 
  evt->set_event_number(m_internal_event_number);
 
  // if using "getGroupWeight" and | lhastrategy | = 4, then need to convert mb to pb ( done otherwise when calling info.weight(), [...] )
  if( m_internal_event_number == 1 && std::abs(m_pythia->info.lhaStrategy()) == 4 ) {
     m_conversion = ( (double) PYTHIA8_CONVERSION);
     ATH_MSG_DEBUG(" LHA strategy needs a conversion to fix Pythia8 shower weights bug(s) equal to " << m_conversion);
  }
 
 
  if(m_pythia->event.size() < 2){
    ATH_MSG_ERROR("Something wrong with this event - it contains fewer than 2 particles!");
    ATH_MSG_ERROR("internal event number is "<<m_internal_event_number);
    return StatusCode::FAILURE;
  }
 
  m_pythiaToHepMC.fill_next_event(*m_pythia, evt, m_internal_event_number);
 
  // In case we're asked, save the LHE event as an attribute.
  if(m_saveLHE){
    auto evtlhe = std::make_shared<HepMC::GenEvent>();
    m_pythiaToHepMC.fill_next_event(m_pythia->process, evtlhe.get(), m_internal_event_number, &m_pythia->info, &m_pythia->settings);
    auto extra = std::make_shared<HepMC::ShortEventAttribute>(evtlhe.get());
    evt->add_attribute(HepMCStr::LHERecord, extra);
  }
 
  // in debug mode you can check whether the pdf information is stored
  if(evt->pdf_info()){
    ATH_MSG_DEBUG("PDFinfo id1:" << evt->pdf_info()->parton_id[0]);
    ATH_MSG_DEBUG("PDFinfo id2:" << evt->pdf_info()->parton_id[1]);
    ATH_MSG_DEBUG("PDFinfo x1:" << evt->pdf_info()->x[0]);
    ATH_MSG_DEBUG("PDFinfo x2:" << evt->pdf_info()->x[1]);
    ATH_MSG_DEBUG("PDFinfo scalePDF:" << evt->pdf_info()->scale);
    ATH_MSG_DEBUG("PDFinfo pdf1:" << evt->pdf_info()->pdf_id[0]);
    ATH_MSG_DEBUG("PDFinfo pdf2:" << evt->pdf_info()->pdf_id[1]);
  }
  else
    ATH_MSG_DEBUG("No PDF information available in HepMC::GenEvent!");
 
  // set the randomseeds
  if(m_useRndmGenSvc) HepMC::set_random_states(evt,m_seeds);
 
  // fill weights
  ATH_CHECK(fillWeights(evt));
 
  return StatusCode::SUCCESS;
}

StatusCode Pythia8_i::fillWeights(HepMC::GenEvent *evt){
 
  // reset
  m_mergingWeight    = m_pythia->info.mergingWeightNLO(); // first initialisation, good for the nominal weight in any case (see Merging:includeWeightInXsection).
  m_enhanceWeight    = 1.0;  // better to keep the enhancement from UserHooks in a separate variable, to keep the code clear
 
#ifndef PYTHIA8_304SERIES
  // include Enhance userhook weight
  for(const auto &hook: m_userHooksPtrs) {
    if (hook->canEnhanceEmission()) {
      m_enhanceWeight *= hook->getEnhancedEventWeight();
    }
  }
#endif // not PYTHIA8_304SERIES
 
  // DO NOT try to distinguish between phase space and merging contributions: only their product contains both, so always multiply them in order to be robust
  double eventWeight = m_pythia->info.weight() * m_mergingWeight * m_enhanceWeight; 
 
  std::map<std::string,double> fWeights; // temporary weight vector, needed while managing hepmc2 and 3 simultaneously
 
  // save as Default the usual pythia8 weight, including enhancements
  // NB: info.weight() is built upon info.eventWeightLHEF (but includes shower "decorations"), not from the rwgt LHE vector.
  size_t atlas_specific_weights = 1;
  fWeights["Default"]=eventWeight;
  if(m_internal_event_number == 1)  {
     ATH_MSG_INFO("found LHEF version "<< m_pythia->info.LHEFversion() );
     if (m_pythia->info.LHEFversion() > 1)  m_doLHE3Weights = true;
     m_weightIDs.clear();
     m_weightIDs.push_back("Default"); // notice that we are assigning a weightname = weightID
  }
 
  // Now systematic weights: a) generator and b) shower variations
  // merging implies reading from lhe files (while the opposite is not true, ok, but still works)
  if (m_lheFile!="") {
       // remember that the true merging component can be disentangled only when there are LHE input events
 
       // make sure to get the merging weight correctly: this is needed  -only- for systematic weights (generator and parton shower variations)
       // NB: most robust way, regardless of the specific value of Merging:includeWeightInXsection (if on, info.mergingWeight() is always 1.0 ! )
       m_mergingWeight *= ( m_pythia->info.weight() / m_pythia->info.eventWeightLHEF ); // this is the actual merging component, regardless of conventions,
                                                                                      // needed by systematic variation weights
                                                                                      // this includes rare compensation or selection-biased weights
  }
 
 
  ATH_MSG_DEBUG("Event weights: enhancing weight, merging weight, total weight = "<<m_enhanceWeight<<", "<<m_mergingWeight<<", "<<eventWeight);
 
  // we already filled the "Default" weight
  std::vector<std::string>::const_iterator id = m_weightIDs.begin()+atlas_specific_weights;
 
  // a) fill generator weights
  if(m_pythia->info.getWeightsDetailedSize() != 0){
    for(std::map<std::string, Pythia8::LHAwgt>::const_iterator wgt = m_pythia->info.rwgt->wgts.begin();
        wgt != m_pythia->info.rwgt->wgts.end(); ++wgt){
 
      if(m_internal_event_number == 1){
        m_weightIDs.push_back(wgt->first);
      }else{
        if(*id != wgt->first){
          ATH_MSG_ERROR("Mismatch in LHE3 weight id.  Found "<<wgt->first<<", expected "<<*id);
          return StatusCode::FAILURE;
        }
        ++id;
      }
 
      std::map<std::string, Pythia8::LHAweight>::const_iterator weightName = m_pythia->info.init_weights->find(wgt->first);
      if(weightName != m_pythia->info.init_weights->end()){
        fWeights[weightName->second.contents] = m_mergingWeight * m_enhanceWeight * wgt->second.contents;
      }else{
        // if no name was assigned/found, assign a weightname = weightid
        fWeights[wgt->first] = m_mergingWeight * m_enhanceWeight * wgt->second.contents;
      }
 
    }
  }
 
  // b) shower weight variations
  // always start from second shower weight: the first was saved already as "Default"
 
  for(int iw = 1; iw < m_pythia->info.PYTHIA8_NWEIGHTS(); ++iw){
 
    std::string wtName = ((int)m_showerWeightNames.size() == m_pythia->info.PYTHIA8_NWEIGHTS())? m_showerWeightNames[iw]: "ShowerWt_" +std::to_string(iw);
 
    fWeights[wtName] = m_mergingWeight * m_enhanceWeight * m_pythia->info.PYTHIA8_WEIGHT(iw)*m_conversion;
 
    if(m_internal_event_number == 1) {
        m_weightIDs.push_back(wtName);
        ATH_MSG_DEBUG("Shower weight name, value, conversion: "<<wtName<<", "<<  fWeights[wtName] <<","<<m_conversion );
    }
  }
 
  // Now save, as a backup, also the LHEF weight without enhancements (useful for possible, rare, non-default cases)
  // to make clear that it should not be used in analyses, save it always with a -10 factor (so that its goal is clear even if not checking its name)
  if (m_lheFile!="") 
  {
       fWeights["EXTRA_bare_LHE_weight"]=(-10.0)*m_pythia->info.eventWeightLHEF;
       if(m_internal_event_number == 1)  m_weightIDs.push_back("EXTRA_bare_LHE_weight");
  }
 
  // Sad, but needed: create a string vector with acceptable order of weight names
  if(m_internal_event_number == 1){
     m_weightNames.clear();
     for(const std::string &id : m_weightIDs){
       if(m_doLHE3Weights) {
         std::map<std::string, Pythia8::LHAweight>::const_iterator weight = m_pythia->info.init_weights->find(id);
         if(weight != m_pythia->info.init_weights->end()) m_weightNames.push_back(weight->second.contents);
         else m_weightNames.push_back(id);
       }
       else { 
     m_weightNames.push_back(id);
       }
     }
     if (m_weightNames.size() !=  fWeights.size() ) {
         ATH_MSG_ERROR("Something wrong when building list of weight names: " << m_weightNames.size() << " vs "<< fWeights.size() << ", exiting ...");
         return StatusCode::FAILURE;
     }
  }
 
  // The following depends on the specific hepmc2/3 implementation
  if (!evt->run_info()) {
     evt->set_run_info(m_runinfo);
  }
  evt->run_info()->set_weight_names(m_weightNames);
 
  // for the first event, weight EXTRA_bare_LHE_weight is not present in evt->weights(), so we need to book a place for it by hand
  if (m_internal_event_number == 1 && evt->run_info()->weight_names().size() == evt->weights().size()+1 ) {
     evt->weights().push_back(1.0);
  }
 
  // added conversion GeV ->  MeV to ensure correct units
  evt->set_units(HepMC3::Units::MEV, HepMC3::Units::MM);
 
  evt->weights().resize(fWeights.size(), 1.0);
  for (const auto & w: fWeights) {
      evt->weight(w.first)=w.second;
  }
 
  auto beams=evt->beams();
  ATH_MSG_DEBUG( " Energy of the beams " << beams[0]->momentum().e() );
 
//uncomment to list HepMC events
//    std::cout << " print::listing Pythia8 " << std::endl;
//    HepMC3::Print::listing(std::cout, *evt); 
 
 
  return StatusCode::SUCCESS;
}

StatusCode Pythia8_i::genFinalize(){
 
  ATH_MSG_INFO(">>> Pythia8_i from genFinalize");
  m_pythia->stat();
 
  Pythia8::Info info = m_pythia->info;
  double xs = info.sigmaGen(); // in mb
 
  if(m_doCKKWLAcceptance){
    const double accfactor = m_nMerged / info.nAccepted();
    ATH_MSG_DEBUG("Multiplying cross-section by CKKWL merging acceptance of "<<m_nMerged <<"/" <<info.nAccepted() << " = " << accfactor
                  << ": " << xs << " -> " << xs*accfactor);
    xs *= accfactor;
  }
 
  if(m_doFxFxXS){
    ATH_MSG_DEBUG("Using FxFx cross section recipe: xs = "<< m_sigmaTotal << " / " << 1e9*info.nTried());
    xs = m_sigmaTotal / (1e9*info.nTried());
    std::cout << "Using FxFx cross section recipe: xs = "<< m_sigmaTotal << " / " << 1e9*info.nTried() << std::endl;
  }
 
  if( ! m_athenaTool.empty()){
    double xsmod = m_athenaTool->CrossSectionScaleFactor();
    ATH_MSG_DEBUG("Multiplying cross-section by Pythia Modifier tool factor " << xsmod );
    xs *= xsmod;
  }
 
  xs *= 1000. * 1000.;//convert to nb
 
  std::cout << "MetaData: cross-section (nb)= " << xs <<std::endl;
  std::cout << "MetaData: generator= Pythia 8." << std::string(py8version()) <<std::endl;
 
  if(m_doLHE3Weights || m_weightIDs.size()>1 ){
    std::cout<<"MetaData: weights = ";
    for (const auto& w : m_weightNames ) std::cout<< w <<" | ";
    std::cout<<std::endl;
  }
 
  if (m_computeEfficiency){
        if (info.nTried()>0) ATH_MSG_INFO("Pythia8 efficiency (nAccepted/nTried %) = " << info.nAccepted()*100./info.nTried());
        else ATH_MSG_INFO("Pythia8 efficiency cannot be computed, nTried <=0");
  }
 
if(m_useRndmGenSvc){
  ATH_MSG_INFO("Number of random numbers produced  " << m_atlasRndmEngine->getRNCalls());
  if (m_useReseed) ATH_MSG_INFO("Each event was reseeded ");
  else  ATH_MSG_INFO("Events were not reseeded ");
}      
 
  return StatusCode::SUCCESS;
}

double Pythia8_i::pythiaVersion()const{
  return m_version;
}

const std::string& Pythia8_i::pythia_stream() {
  return s_pythia_stream;
}

std::string Pythia8_i::xmlpath(){
 
  
  std::string foundpath = "";
 
// Try to find the xmldoc directory using PathResolver:
  foundpath = PathResolverFindCalibDirectory( "Pythia8/xmldoc" );
 
  return foundpath;
}

 
// fix Pythia8 shower weights change in conventions
#ifdef PYTHIA8_NWEIGHTS
  #undef PYTHIA8_NWEIGHTS
  #undef PYTHIA8_WEIGHT
  #undef PYTHIA8_WLABEL
  #undef PYTHIA8_CONVERSION
#endif