The chemical stage of theMonte Carlo track-structure simulation codeGeant4-DNA has been revised andvalidated. The root-mean-square (RMS) empirical parameter that dictates the displacement ofwatermolecules after an ionization and excitation event inGeant4-DNA has been shortened to betterfitexperimental data. The pre-defined dissociation channels and branching ratioswere notmodified, but thereaction rate coefficientsfor simulating the chemical stage ofwater radiolysiswere updated. The evaluationofGeant4-DNA was accomplishedwith TOPAS-nBio. For that,we compared predicted time-dependentGvalues in pure liquidwaterfor·OH, e–aq, and H2with published experimental data. For H2O2 and H·,simulation of added scavengers at different concentrations resulted in better agreementwithmeasurements.In addition, DNA geometry information was integratedwith chemistry simulation in TOPAS-nBio to realizereactions between radiolytic chemical species and DNA. Thiswas used in the estimation of the yield of singlestrand breaks(SSB)induced by 137Cs γ-ray radiolysis of supercoiled pUC18 plasmids dissolved in aeratedsolutions containing DMSO. The efficiency of SSB induction by reaction between radiolytic species andDNA used in the simulation was chosen to provide the best agreementwith publishedmeasurements. AnRMS displacement of 1.24 nm provided agreementwithmeasured datawithin experimental uncertaintiesfor time-dependentGvalues and under the presence of scavengers. SSB efficiencies of 24% and 0.5%for·OHand H·, respectively, led to an overall agreement of TOPAS-nBio resultswithin experimental uncertainties.The efficiencies obtained agreed with values obtainedwith published non-homogeneous kineticmodel andstep-by-stepMonte Carlo simulations but disagreed by 12%with published directmeasurements.Improvement of the spatial resolution of the DNA damagemodelmightmitigate such disagreement. Inconclusion,with these improvements,Geant4-DNA/TOPAS-nBio provides afast, accurate, and userfriendly toolfor simulating DNA damage under low linear energy transfer irradiation.< Réduire