Purpose: Simulation of indirect damage originating from the attack of free radical species producedby ionizing radiation on biological molecules based on the independent pair approximation is investigated in this work. In addition, a new approach, relying on the independent pair approximation that isat the origin of the independent reaction time (IRT) method, is proposed in the chemical stage ofGeant4-DNA.Methods: This new approach has been designed to respect the current Geant4-DNA chemistryframework while proposing a variant IRT method. Based on the synchronous algorithm, this implementation allows us to access the information concerning the position of radicals and may make itmore convenient for biological damage simulations. Estimates of the evolution of free species as wellas biological hits in a segment of DNA chromatin fiber in Geant4-DNA were compared for thedynamic time step approach of the step-by-step (SBS) method, currently used in Geant4-DNA, andthis newly implemented IRT.Results: Results show a gain in computation time of a factor of 30 for high LET particle tracks witha better than 10% agreement on the number of DNA hits between the value obtained with the IRTmethod as implemented in this work and the SBS method currently available in Geant4-DNA.Conclusion: Offering in Geant4-DNA more efficient methods for the chemical step based on theIRT method is a task in progress. For the calculation of biological damage, information on the position of chemical species is a crucial point. This can be achieved using the method presented in thispaper< Réduire