36.6. Common Physics Lists

Table of Contents

Overview

Geant4 provides many ready to use, preconfigured physics lists. These lists bundle particle definitions and processes into combinations that are suitable for particular energy ranges and applications. Using a reference list is usually much easier and more reliable than building your own from scratch.

This appendix chapter collects the most commonly used reference physics lists, describes their typical use cases, and gives simple guidance on how to select one for a beginner level simulation.

Important rule: For most beginner applications you should use a standard reference physics list instead of writing a custom one. Start with a recommended list, validate your results, and only customize when you clearly understand what needs to change.

Naming Conventions

Reference physics lists are provided in the Geant4 source tree and follow a naming pattern that encodes the hadronic model combination and extra options. Names are uppercase with underscores, such as FTFP_BERT or QGSP_BIC_HP.

The first part usually indicates the high energy hadronic model, for example:

Prefix	Meaning	Typical high energy range
FTFP	Fritiof string model with precompound	Above a few GeV up to many tens of GeV
QGSP	Quark Gluon String model with precompound	Similar high energy range

The middle part indicates the low or intermediate energy hadronic model:

Suffix block	Meaning	Typical low energy range
BERT	Bertini intra nuclear cascade	Up to a few GeV
BIC	Binary cascade	Up to about 10 GeV
INCLXX	Liège cascade	Up to a few GeV

Additional suffixes indicate special options:

Extra suffix	Meaning
_HP	High precision neutron models, important below 20 MeV
_EMV, _EMZ	Alternative electromagnetic options
_LIV	Livermore low energy EM models
_PEN	Penelope style low energy EM models

Not every combination exists. You should only use physics lists that are actually distributed with your Geant4 version.

Widely Used General Purpose Lists

For many detector simulations you need a broad energy range and a mix of electromagnetic, hadronic, and decay processes. The lists in this section are designed to be good default choices.

FTFP\_BERT

FTFP_BERT is the current standard reference list for many applications and is the default recommendation in the Geant4 manual for a wide range of use cases.

It combines the FTFP high energy hadronic model with the Bertini cascade at lower energies, and a standard electromagnetic configuration. At the time of writing, this list is actively maintained and regularly tested against data.

Typical uses include calorimeter simulations, high energy physics detectors, shielding, and general purpose radiation transport where neutrons below 20 MeV are not the central focus.

Recommended default: If you do not know which physics list to choose for a general detector or shielding simulation, start with FTFP_BERT.

QGSP\_BERT

QGSP_BERT is an older but still frequently used general purpose list. It uses the QGSP model at high energy, plus Bertini at lower energies, and standard EM.

Some older experiments and validation studies are based on this list. You may encounter it in example codes, publications, or legacy projects.

For new designs Geant4 developers usually recommend FTFP_BERT over QGSP_BERT. However, if you need to compare with older simulations that used QGSP_BERT it is still available and supported.

FTFP\_BIC and QGSP\_BIC

FTFP_BIC and QGSP_BIC use the Binary cascade for low and intermediate hadronic energies. The Binary cascade is often preferred for detailed nucleon and light ion interactions in the low GeV range.

These lists are often used for studies that are sensitive to hadron scattering and reaction products around a few hundred MeV to a few GeV, such as some medical and space applications. For a beginner, they are mainly relevant when instructions or publications explicitly mention them.

Lists with High Precision Neutrons

If your application depends on accurate transport of neutrons below about 20 MeV, you need a list that uses the high precision, or HP, neutron models. These models rely on evaluated nuclear data files and treat neutron interactions in detail.

FTFP\_BERT\_HP

FTFP_BERT_HP is similar to FTFP_BERT but adds high precision models for low energy neutrons. This is useful for shielding and activation studies where slow neutron capture and scattering are important.

The HP treatment increases memory usage and CPU time compared to the non HP version. You should only choose the HP variant if you actually need detailed low energy neutron physics.

QGSP\_BIC\_HP and QGSP\_BERT\_HP

QGSP_BIC_HP and QGSP_BERT_HP combine the HP neutron option with either the Binary or Bertini cascades. These lists are often used in radiation protection, reactor related studies, or where precise neutron dose and spectra are required.

If you are simulating a problem where neutrons are a dominant component and their slowing down and capture matter, an HP list is usually the conservative choice.

Rule of thumb: If your results depend strongly on neutrons below 20 MeV, use a physics list with the _HP suffix and make sure the required neutron data libraries are installed.

Electromagnetic Low Energy Options

Standard reference lists include a default electromagnetic configuration that is usually sufficient for many high energy applications. However, there are special EM options for low energy photons and electrons, where detailed atomic effects are important.

These EM variations are often attached to a hadronic base like FTFP_BERT. Here we focus on the EM parts, independently of the hadronic block.

Livermore EM lists

Lists that include Livermore models typically use a suffix such as _LIV or include Livermore physics in a custom configuration.

The Livermore models provide detailed low energy electromagnetic processes for photons and electrons. They go down to a few hundred eV and include fine structure such as detailed photoelectric and Compton cross sections and atomic shell structure.

They are particularly useful in medical and microdosimetry simulations, or when dose and interaction patterns at very low energies or in small structures are important.

Penelope EM lists

Lists based on the Penelope style EM models often have the suffix _PEN or are built by including the Penelope EM physics constructor.

Penelope models are also optimized for low and intermediate energy electron and photon transport. They are used in some medical physics and radiation physics studies. The choice between Livermore and Penelope is often based on validation studies for a given application.

For a beginner, you usually encounter these EM options via a prepared physics list in an example or a publication. Switching from the default EM to a low energy EM option is a targeted change for specific accuracy needs.

Important statement: Only switch to low energy EM options such as Livermore or Penelope when your study requires detailed physics below a few tens of keV, and verify that your cuts and geometry scales are consistent with this choice.

Specialized Application Lists

Some physics lists are tailored for specific application domains. In many cases they differ from the general lists by the choice of production cuts, extra processes, or tuned configurations for performance versus accuracy.

Shielding related lists

For shielding and radiation protection there are lists like Shielding and ShieldingLEND that are configured to model deep shielding problems and neutron transport efficiently.

These lists combine suitable hadronic models, in many cases with HP options or alternative neutron data handling, and are tuned for large, thick shields where secondary neutrons are important.

If your project is primarily about dose outside thick shielding or activation behind shields, these specialized lists can be helpful.

Medical and dosimetry oriented lists

There are also reference configurations intended for medical or dosimetry simulations, for example in the extended examples. These may not always appear as a single name in the same way as the global lists, but they embody common choices such as low energy EM options, Binary cascade, and HP neutrons, together with appropriate cuts.

For beginners, it is often best to start from an official medical physics example that already chooses a suitable physics configuration, then adapt geometry and sources as needed.

Space and radiation effects lists

For space and electronics radiation effects, lists often emphasize hadronic accuracy through the atmosphere and in materials, and can include detailed ion and fragment physics. Again, examples in the Geant4 distribution provide concrete configurations, often extending a standard base such as FTFP_BERT.

How to Select and Use a Physics List

Selecting a physics list is always a compromise between accuracy, CPU time, and memory. The following simple guidelines can help beginners:

First, decide if neutrons below 20 MeV are critical. If they are essential for your observables, choose a list with _HP. If not, start with a non HP list to save computing resources.

Second, decide if very low energy electromagnetic physics below a few tens of keV is critical in small volumes. If you are simulating dosimetry in microscopic structures, semiconductor devices, or low energy x ray interactions, use a list or configuration that includes Livermore or Penelope EM. If you work mainly with MeV scale gamma and charged particle interactions in larger detectors, the default EM is often adequate.

Third, for general high energy detector simulations, calorimetry, and many beginner examples, select FTFP_BERT. For legacy comparison, use the same list as in the original study, often QGSP_BERT or another combination explicitly mentioned.

Fourth, for thick shielding or reactor like problems, prefer a shielding oriented list or an HP list combined with validation against reference data.

Selection summary:

Start with FTFP_BERT for a wide range of detectors.
Switch to _HP variants if slow neutrons are important.
Switch to Livermore or Penelope EM only if you need detailed low energy EM.
Always validate your choice against experimental data or trusted calculations.

Once chosen, you typically use a reference physics list by instantiating it in your PhysicsList or ActionInitialization code, often through factory functions provided by Geant4. The exact code usage is covered elsewhere in the course, but conceptually, the list is a single object that registers all required particle definitions and processes with the Geant4 kernel.

Summary Table of Common Physics Lists

The table below summarizes typical common lists and their main intended uses. Exact behavior can change between Geant4 versions, so always consult the version specific documentation.

Physics list	Main features	Typical use case
FTFP_BERT	General hadronic + standard EM	Default for many HEP and detector simulations
QGSP_BERT	Older general list with QGSP + Bertini	Legacy simulations, comparison with old results
FTFP_BERT_HP	FTFP_BERT with high precision neutrons	Shielding, neutron sensitive applications
QGSP_BIC	QGSP + Binary cascade	Hadron therapy, low to mid energy hadrons
QGSP_BIC_HP	QGSP_BIC with high precision neutrons	Detailed neutron and proton dosimetry
Shielding	Optimized for deep shielding and neutron transport	Radiation protection, shielding design
Lists with _LIV	Livermore low energy EM models	Low energy x ray, microdosimetry, medical EM
Lists with _PEN	Penelope style low energy EM models	Detailed EM at low energies, certain medical apps

This appendix should help you recognize the most common physics list names you will encounter in examples, publications, and other chapters of this course, and give you a first idea of which one is suitable for your own beginner simulations.