ATLAS Monte Carlo generator tunes to LHC data

Tunes of the PYTHIA and HERWIG/JIMMY Monte Carlo event generators to the first measurements from the LHC are presented, based on data collected at center-of-mass energies of 900 GeV and 7 TeV. Measurements of the charged particle multiplicities and of the momentum flow in the underlying event in inelastic pp collisions are used to constrain the parameters of multiple parton scattering models. The tunes aim to consistently describe both the new measurements and pre-LHC data from the Tevatron and LEP, but some tension is seen between the datasets for both models. We conclude by noting the modelling issues still requiring attention to produce comprehensive event generator tunings for use in LHC new physics searches at 7 TeV.