Schematic illustration of the modeling process. In step 1, the chemistry in a circumstellar disk model is evolved for 4 Myr. This chemistry is extracted from the gap outer wall region and used as a starting point prior to accretion. To consider various possible accretion scenarios, the composition of the infalling material is either reset to atomic (full reset), the ices are sublimated (partial reset) or the chemistry remains unaltered (inherit). In step 2, the chemistry of a parcel of gas and ice is evolved for 10 yr as it travels towards the CPD midplane. In step 3, the chemistry is evolved at the CPD midplane for the viscous timescale of the disk.