  The accurate simulation of additional interactions at the ATLAS experiment
for the analysis of proton-proton collisions delivered by the Large Hadron
Collider presents a significant challenge to the computing resources. During
the LHC Run (2015-2018) there were up to 70 inelastic interactions per bunch
crossing, which need to be accounted for in Monte Carlo (MC) production. In
this document, a new method to account for these additional interactions in the
simulation chain is described. Instead of sampling the inelastic interactions
and adding their energy deposits to a hard-scatter interaction one-by-one, the
inelastic interactions are presampled, independent of the hard scatter, and
stored as combined events. Consequently, for each hard-scatter interaction only
one such presampled event needs to be added as part of the simulation chain.
For the Run 2 simulation chain, with an average of 35 interactions per bunch
crossing, this new method provides a substantial reduction in MC production CPU
needs of around 20%, while reproducing the properties of the reconstructed
quantities relevant for physics analyses with good accuracy.
