You may have some of the following packages already installed on your system.

1. Install Xcode 11 from the App Store.

2. Install Xcode command-line tools:

   xcode-select --install
   

3. Install additional packages via homebrew:

   brew install cmake
   brew install doxygen
   brew install root6
   brew install graph-tool
   brew install hdf5
   brew install open-mpi
   brew install gsl
   brew install boost
   brew install zlib
   brew tap brewsci/science
   brew install pythia
   

   Activate your ROOT installation by adding the following line to your .bashrc: . /usr/local/bin/thisroot.sh

   Note that brew installs python as dependencies of some of the above packages (namely, python3 with root6, and python2 with graph-tool)

4. Download and install HepMC >=3.1

   curl https://hepmc.web.cern.ch/hepmc/releases/HepMC3.2.0.tgz
   tar -xvzf HepMC3.2.0.tgz 
   cd HepMC3.2.0/cmake
   cmake ..
   make all install
   

   The default path to install HepMC is /usr/local/, which should be automatically found by cmake.

If you encounter any problems, please post an Issue.