How to use gridpacks for NLO event generation?

Created by Rikkert Frederix
Keywords:
NLO gridpack
Last updated by:
Rikkert Frederix

There is no special gridpack mode for NLO event generation.

However, you can easily create a "gridpack" yourself. What you have to do is the following:

1.) Generate and output the process as you normally do.
2.) When you do the launch of the NLO event generation, set the parameter in 'nevents' to zero (in the run_card) and the parameter req_acc to 0.001 (also in the run_card). This will skip the event generation step, but does create the grids with a 0.001 relative precision (basically enough for infinite number of events) that can be used to generate events later on. If you want to generate less than a total of 1M events, you can increase the req_acc to 1/sqrt(number of events) so that the grid setup step is slightly faster. Once the run is done, you have a "gridpack". (Just tar the whole MG5_aMC directory, including the process directory).
3.) From the grids that you just generated, you can generate any small number of events in many bunches "by hand":
a.) set the nevents parameter in the run_card back to the number of events you want to generate in the bunch
b.) set the iseed parameter (seed for the random number generator) in the run_card to a non-trivial number
c.) execute the command:

./bin/generate_events --parton --nocompile --only_generation

and this will generate the small number of events from the grids created before.

Note that you probably want to run the event generation with the gridpack on a single CPU core (and not on multi-core or in cluster mode). This can be changed in the Cards/amcatnlo_configuration.txt file.