Abstract: The concept of the ARAPUCA device is relatively new and involves increasing the effective area for photon collection of SiPMs by the use of a box with highly reflective internal walls, wavelength shifters, and a dichroic filter to allow the light to enter the box and not the leave it. There were a number of tests showing the good performance of this device. Recently an improvement on the original design was proposed: the inclusion of a WLS bar inside the box to guide photons more efficiently to the SiPMs. We present a full simulation of the device using Geant4. We have included all the material properties that are available in the literature and the relevant detailed properties for adequate photon propagation available in the framework. Main results include estimates of detection efficiency as a function of the number, shape, and placing of SiPMs, width of the WLS bar, its possible attenuation, and the existence of a gap between the bar and the SiPMs. Improvement on the efficiency with respect to the original ARAPUCA design is 15-40%. The ARAPUCA simulation has been validated in a number of experimental setups and is a useful tool to help making design choices for future experiments devices Ver menos