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Configuration XML

GENIE manages much of its configuration via XML text files and specific conventions. The compiler cannot help you find problems if you are using a non-sensical configuration file or have typo in your specification XML. On the other hand, the wide use of XML enables a very loose coupling between GENIE components, making maintenance easier, and allows for dynamic run-time flexibility and decision-making. The secret is to understand and use the XML configuration correctly and make heavy use of grep and other tools to look for typos inside the strings.

The first file to examine is config/master_config.xml. This file maps GENIE algorithms and modules to the XML files they need. The files referenced below (e.g., EventGenerator.xml, etc.) are not themselves hard-coded into GENIE but are configurable. Here we present the defaults, but you can change the XML file names in master_config.xml and quickly switch between very large sets of configuration changes.

Important Files

Physics Parameters

  1. config/UserPhysicsOptions.xml: Contains important physical parameters (e.g. CKM matrix parameters, Axial Mass for quasi-elastic scattering, etc.).

UserPhysicsOptions.xml is a good place to start if you want to change something about the physics in GENIE, but you aren’t sure where to look first. It also contains some model execution options (e.g., include lepton mass correction in charged-current coherent pion production or not, etc.). We also often set the algorithm names for implementing cross section models here, e.g.

<param type="alg" name="XSecModel@genie::EventGenerator/COH-CC">
  genie::ReinSeghalCOHPiPXSec/Default
</param>
<param type="alg" name="XSecModel@genie::EventGenerator/COH-NC">
  genie::ReinSeghalCOHPiPXSec/Default
</param>

To replace the ``COH-CC'' model with a different one, we supply a different string here, e.g.:

<param type="alg" name="XSecModel@genie::EventGenerator/COH-CC">
  genie::BergerSehgalCOHPiPXSec/Default
</param>
<param type="alg" name="XSecModel@genie::EventGenerator/COH-NC">
  genie::BergerSehgalCOHPiPXSec/Default
</param>
Event Generator Processes

  1. config/EventGeneratorListAssembler.xml: Contains physics lists.

  2. config/EventGenerator.xml: For each process specified in config/EventGeneratorListAssembler.xml, the list of event record visitors is specified. An event record visitor implements the interface EventRecordVisitorI and the execution process will loop over all of these in order and call the ProcessEventRecord method. Because this list may specified here in XML, we can set the list of computations dynamically and flexibly.

In general, you will want to use a comprehensive list for physics work, but for development, it can be very useful to specify a restricted list like:

<param_set name="CCMEC">
  <param type="int" name="NGenerators"> 1                            </param>
  <param type="alg" name="Generator-0"> genie::EventGenerator/MEC-CC </param>
</param_set>

Here, NGenerators is accessed in EventGeneratorList::AssembleGeneratorList() to count the number of event generation algorithms to load.

The lists of event record visitors configure the actual modules that run while simulating an event. The mechanism is very flexible - you may have any number of modules and you may factorize the total event simulation however necessary. The lists themselves look like:

<param_set name="MEC-CC">
   <param type="string" name="VldContext"> </param>
   <param type="int"    name="NModules">   5              </param>
   <param type="alg"    name="Module-0">
      genie::InitialStateAppender/Default                 </param>
   <param type="alg"    name="Module-1">
      genie::VertexGenerator/Default                      </param>
   <param type="alg"    name="Module-2">
      genie::MECGenerator/Default                         </param>
   <param type="alg"    name="Module-3">
      genie::HadronTransporter/Default                    </param>
   <param type="alg"    name="Module-4">
      genie::UnstableParticleDecayer/AfterHadronTransport </param>
   <param type="alg"    name="ILstGen">
      genie::MECInteractionListGenerator/CC-Default       </param>
  </param_set>

The algorithms are specified as name/configuration'' - this informaiton is parsed when the algorithm is instantiated in AlgFactory::GetAlgorithm(string name, string config). The strings are not arbitrary - check, for example, EventGenerator::LoadConfig() to see how GENIE looks for algorithms keyed by Module-''.