A set of physics and engineering codes is currently used and developed at Consorzio RFX to simulate the negative ion beam accelerators of the next experiments SPIDER and MITICA. In order to verify the accuracy of the simulations, an experimental validation has been undertaken, that consist in benchmarking the codes with data from ITER-relevant experiments. As a first step, the SLACCAD code have been applied to simulate the beam optics in the MANITU testbed at IPP, which features a Radio Frequency ion source and is able to operate with ITER-like conditions in terms of extracted ion current density and pulse length. A description of the modeling activities and a discussion of the main results obtained so far are here reported. In particular, the effect of beam halo, space charge compensation and particle angle at the emitter are investigated. Some results will be used to identify reasonable operating parameters as inputs for the design optimization of the SPIDER and MITICA experiments.