source: HiSusy/trunk/Pythia8/pythia8170/rootexamples/README @ 1

This directory contains files contributed by Rene Brun and 
Axel Naumann, which illustrate how ROOT commands can be used 
inside PYTHIA code. You should read the "ROOT usage" page 
in the manual to understand the function of the files, and 
how to set up the environment in which they can be used.

The practical steps to be taken, assuming you use the bash shell, 
are:

1) Add to your ".bashrc" file (in your home directory) 
the following lines:

########################################################
# Root path and libraries:
source /Path_to_ROOT/root/bin/thisroot.sh
 
# Pythia 8 path and libraries:
export PYTHIA8=/Path_to_Pythia8/pythia81xx/
export LD_LIBRARY_PATH=$PYTHIA8/lib:$LD_LIBRARY_PATH
########################################################

where xx is the Pythia 8.1 subversion.
As usual type "source .bashrc" after the changes.

2) Before compilation, configure Pythia 8 with shared libraries:

./configure --enable-shared 

3) "make hist"  builds an application for histogramming 
Pythia output (alternatively "make" builds and runs all examples).

4) "./hist" runs the example showing the Pythia output histogram 
in Root format.

5) You can correspondingly try the "tree" program, which illustrates
how Pythia event records can be saved by Root. Needless to say, files
can become quite large when many events are generated.

6) When upgrading to a new Pythia subversion you should re-generate 
pythiaDict.cxx to reflect the new classes:

cd NewPythiaVersion/rootexamples
make pythiaDict.cxx

and use the generated pythiaDict.h and pythiaDict.cxx for building 
your program.

7) For FastJet use, add the following lines to the Makefile:

CXXFLAGS += -I/Path_to_FastJet/include

-L$(PYTHIA8)/lib -lpythia8 -llhapdfdummy -L/Path_to_FastJet/lib -lfastjet

where "Path_to_FastJet" is the directory where FastJet was installed.
Finally, add to your ".bashrc" file the following line:

export LD_LIBRARY_PATH=//Path_to_FastJet/lib:LD_LIBRARY_PATH

and type "source .bashrc" after the changes.