In GEANT4, a physics list represents a collection of models describing various phenomena. Specificaly, for each particle, a physics list defines the relevant physical models and their validity range. Geant4 comes with several reference physics lists.

• In AtRIS, the physics list is passed as the first argument

  bash | passing the physics list as an argument

   HOSTNAME$ ./AtRIS FTFP_BERT_HP example -1 

• The authors recommend the use of the current Geant4 default physics list in conjunction with the high precision neutron package (FTFP_BERT_HP). Geant4 collaboration recommends this physics lists for applications dealing with cosmic rays, among others.
• Recent versions of GEANT4 have modular physics lists. Each name element is separated with an underscore from the other. Each name element defines a greater collection of models. For example FTFP_BERT_HP is comprised of
  • the FTFP (Precompound Fritof parton model FTF),
  • the BERT (Bertini style intranuclear cascade model), and
  • the HP (High Precission Neutron Package model).
• The modularity of the physics list makes it possible to replace the “FTFP” and the “BERT” parts with something else. For example, to use the BIC (binary cascade model) instead of BERT, we pass FTFP_BIC_HP as an argument.
• Turning on the HP package considerably increases the time required to perform a simulation.
• All physics lists come with the default electromagnetic constructor (set of physical models). The default constructor represents a balance between precision and performance. It can be replaced by appending an additional suffix at the end of the physics list. For example EM option4, having the suffix EMZ, represents the highest precision electromagnetic physics models. It can be activated by passing FTFP_BERT_HP_EMZ as physics list. The use of this option further prolongs the simulation time.
• Recently, the GEANT4 collaboration has produced a new class of documentation under the title “Guide for Physics List”. See here.
• The best resources when searching for in-depth information about the physical processes, please consider the physics reference manual.
• Additional, more specific modifications of the default physics lists can be achieved by passing standard geant4 macro commands through the simulation macro file.

Compare results from the different electromagnetic constructors
Compare neutron and muon spectra for the different physics lists
Compare the effect that _HP has on the neutron spectra at surface