Differences between HepMC2 and HepMC3

The following is a list of main differences that should be taken into account when transitioning from HepMC2 to HepMC3.

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Technical changes

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Structure change and header file organization

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Following changes in header files have been applied:

HepMC/HeavyIons.h    ->  HepMC3/GenHeavyIons.h   (now a POD struct)
HepMC/PdfInfo.h      ->  HepMC3/GenPdfInfo.h     (now a POD struct)
HepMC/SimpleVector.h ->  HepMC3/FourVector.h     (ThreeVector class removed)

The structure of GenCrossSection class has been changed to handle multiple values of cross-sections. The cross-section values and errors (uncertainties) can be accessed only by public functions, the corresponding data members are private. By default, the number of cross-section values in every event is equal to the number of event weights. Accordingly, each cross-section value can be accessed using the corresponding event weight name (std::string) or event weight index (int).

Following header files are no longer available:

CompareGenEvent.h
Flow.h
GenRanges.h
HepMCDefs.h
HerwigWrapper.h
IteratorRange.h
Polarization.h
SearchVector.h
StreamHelpers.h
StreamInfo.h
TempParticleMap.h
WeightContainer.h
IO_GenEvent.h
 
enable_if.h
is_arithmetic.h
 
IO_AsciiParticles.h
IO_Exception.h
IO_HEPEVT.h
IO_HERWIG.h
 
PythiaWrapper.h
PythiaWrapper6_4.h
PythiaWrapper6_4_WIN32.h

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Fortran generators

########################################################################### An example of interface to Pythia6 Fortran blocks is given in the examples.

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Changes to the I/O handeling

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Multiple file formats are supported. The implementation of reading and wrining is separated in HepMC3. All the reading operations are performed in "reader" objects inherited from HepMC::Reader and the writing operations in the "writer" objects inherited from HePMC::Writer. Therefore it is to use the desired headers explicitely, as needed.

The IO_GenEvent.h header is not available anymore. to write and/or read HepMC2 files the following includes

WriterAsciiHepMC2.h
ReaderAsciiHepMC2.h

should be used instead of

IO_GenEvent.h

Please note that HepMC2 format is outdated and is not able to contain a lot of information stored into event record by the modern Monte Carlo event generators. It is recommended to use HepMC3 native event record in plain text or in ROOT TTree format. The corresponding readers and writers are

WriterAscii.h
ReaderAscii.h

and

WriterRootTree.h
ReaderRootTree.h

Implementation of custom Reader and Writer objects is possible as well.

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Memory managed by shared pointers

########################################################################### Particles and vertices are managed using shared pointers, so they should not be created through the call to 'new'.

GenEvent event;
 
// Create particles
GenParticlePtr p1 = make_shared<GenParticle>();
GenParticlePtr p2 = make_shared<GenParticle>();
GenParticlePtr p3 = make_shared<GenParticle>();
 
p1->set_pdg_id(23);
p2->set_pdg_id(15);
p3->set_pdg_id(-15);
 
// Create vertex
GenVertexPtr v1 = make_shared<GenVertex>();
v1->add_particle_in(p1);
v1->add_particle_out(p2);
v1->add_particle_out(p3);
 
event.add_vertex(v1);

Note

An interface to convert raw pointers to GenParticlePtr, GenVertexPtr is available for backward-compatibility. It is marked as deprecated.

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Iterators

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The iterator-bases classe and access functions from HepMC2, e.g.

class particle_iterator;
class vertex_iterator;
class edge_iterator;
...
inline int GenEvent::particles_size() const;
inline int GenEvent::vertices_size() const;
...

were removed. The C++11 itarations should be used instead, e.g. instead of

for (GenEvent::particle_const_iterator p = evt->particles_begin(); p != evt->particles_end(); ++p ) {
...
}

one should use

for (auto p: evt->particles()) {
...
}

or alternatively

for (size_t i=0;i<evt->particles().size();++i) {
...
evt->particles().at(i)
...
}

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Topological order

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Particles and vertices in HepMC3 are stored in topological order. This means that when creating vertices, incoming particles must have id lower than any of the outgoing particles.

This forces the tree structure to be constructed top-to-bottom and disallows creating loops.

GenParticlePtr p1 = make_shared<GenParticle>();
GenParticlePtr p2 = make_shared<GenParticle>();
GenParticlePtr p3 = make_shared<GenParticle>();
GenParticlePtr p4 = make_shared<GenParticle>();
 
GenVertexPtr v1 = make_shared<GenVertex>();
GenVertexPtr v2 = make_shared<GenVertex>();
GenVertexPtr v3 = make_shared<GenVertex>();
 
event.add_particle(p1);
event.add_particle(p2);
event.add_particle(p3);
event.add_particle(p4);
event.add_vertex(v1);
event.add_vertex(v2);
event.add_vertex(v3);
 
v1->add_particle_in (p2);
v1->add_particle_out(p3);
v1->add_particle_in (p4); // will cause error, because p3
                          // has higher index than p4
 
v2->add_particle_in (p4);
v2->add_particle_out(p3); // will also cause error
 
// Order of vertices does not matter. Index of end vertex
// can be lower than index of production vertex
v3->add_particle_in (p1);
v3->add_particle_out(p2);

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Changes to user interface and to HepMC functionality

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Deleting particles and vertices

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Deleting a particle using GenEvent::remove_particle() will also remove its end_vertex if this is the only particle that is on this vertex particles_in() list.

Deleting a vertex will delete all of its outgoing particles. (and subsequently, all of their decays).

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Barcodes can no longer be set

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Barcodes are no longer available. Use attributes to provide additional information that was previously encoded using barcodes (see module Standard attributes).

The unique identifier of particles and vertices is now called id() to separate its role from barcodes. Id is set automatically and cannot be changed. Id is not permanently attached to particle/vertex. When a particle or vertex is removed from the event, id's of other particles or vertices may change.

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Units are no longer defined at compilation time

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The default units are set to GEV and MM. They can be provided as constructor parameters or changed later using HepMC::GenEvent::set_units

GenEvent event(Units::GEV,Units::CM);
 
GenParticlePtr p = make_shared<GenParticle>();
 
event.add_particle(p);
...
 
event.print(); // event printed in GEV/CM
 
event.set_units(Units::MEV,Units::MM); // will trigger unit conversion for all particles and vertices
 
event.print(); // event printed in MEV/MM

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Deprecated code

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A lot of HepMC2 functions has been declared obsolete and are marked as deprecated. Warnings displayed at compilation time hint to what functions or classes should be used instead.

// HepMC2 code:
HepMC::FourVector position(pos[1],pos[2],pos[3],pos[0]);
vertex = new HepMC::GenVertex(position, id);
 
// Replace with:
HepMC3::FourVector position(pos[1],pos[2],pos[3],pos[0]);
HepMC3::GenVertexPtr vertex = std::make_shared<HepMC3::GenVertex>(position);
vertex->set_id(1);

// HepMC2 code:
std::vector<HepMC::GenParticle*> beamparticles
// ...
event.set_beam_particles(beamparticles[0],beamparticles[1]);
 
// Replace with:
std::vector<HepMC3::GenParticlePtr> beamparticles;
// ...
event.add_particle(beamparticles[0]);
event.add_particle(beamparticles[1]);

// HepMC2 code:
HepMC::GenVertex * vertex;
vertex->set_id(1);
vertex->id();
 
// Replace with:
HepMC3::GenVertexPtr vertex = std::make_shared<HepMC3::GenVertex>();
vertex->set_status(1);
vertex->status();

// HepMC2 code:
HepMC::GenVertex * vertex;
for (HepMC::GenVertex::particles_out_const_iterator pout
       =v->particles_out_const_begin();
     pout!=(*vit)->particles_out_const_end(); ++pout) { }
 
// Replace with (similarly for particles_in):
HepMC3::GenVertexPtr vertex = std::make_shared<HepMC3::GenVertex>();
for (HepMC3::GenParticlePtr pout : vertex->particles_out() ) { }

// HepMC2 code:
vertex->weights().push_back(1.);
// Replace with:
TODO

GenEvent evt(Units::GEV,Units::MM);
// HepMC2 code:
evt.set_alphaQCD(m_alphas);
evt.set_alphaQED(m_alpha);
evt.set_event_scale(m_event_scale);
evt.set_mpi(m_mpi);
evt.set_signal_process_id(m_signal_process_id);
evt.set_random_states(m_random_states);
// Replace with:
evt.add_attribute("alphaQCD",
                   make_shared<DoubleAttribute>(m_alphas));
evt.add_attribute("alphaEM",
                   make_shared<DoubleAttribute>(m_alpha));
evt.add_attribute("event_scale",
                   make_shared<DoubleAttribute>(m_event_scale));
evt.add_attribute("mpi",
                   make_shared<IntAttribute>(m_mpi));
evt.add_attribute("signal_process_id",
                   make_shared<IntAttribute>(m_signal_process_id));
for ( size_t i=0;i<m_random_states.size();i++)        
  evt.add_attribute("random_states"+to_string(i),make_shared<IntAttribute>(m_random_states[i]));

// HepMC2 code:
HepMC::GenVertex * vertex;
...
vertex->weights().push_back(1.);
// Replace with:
vertex->add_attribute("weight1", make_shared<DoubleAttribute>(1.0));

// HepMC2 code:
HepMC::GenVertex * vertex;
vertex->check_momentum_conservation();
// Replace with:
TODO

// HepMC2 code:
HepMC::GenParticle::set_flow(int code_index, int code = 0)  
HepMC::GenParticle::set_polarization( Polarization(theta,phi))  
// Replace with:
HepMC3::GenParticle::add_attribute("flow"+to_string(code_index),make_shared<IntAttribute>(code));
HepMC3::GenParticle::add_attribute("theta",make_shared<DoubleAttribute>(theta));
HepMC3::GenParticle::add_attribute("phi",make_shared<DoubleAttribute>(phi));

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Standard attributes

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For the user convenience and backward compatibility the following standard attributes are supported for the

GenEvent

double alphaQCD
double alphaEM
double event_scale
int mpi        
int signal_process_id
int signal_vertex_id
int random_states1... random_statesN

GenVertex

double weight1... weightN

GenParticle

int flow
double theta
double phi

The presence of cycles in the event structure is indecated with an attribute

int cycles

Note that attributes belong to the event, therefore these can be set only for particles and vertices that belong to a GenEvent object.

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Last update 24 Jan 2019