$t_1$ in physical conditions corresponding to the core extension (stage 1), and then the protostellar core model including the low-density extension was run over time $t_2$ (stage 2) using the initial chemical abundances obtained from the stage 1. We explored a range of $t_1$ and $t_2$ values to search for the best fit to the observed column densities.

![image](bestfit_IRAS16293_multilayer_5e5.pdf){width="2.0\columnwidth"}

The simulation results depend on how the chemical reactions related to (deuterated) amidogen formation are assumed to proceed. Here we used our gas-grain chemical code [@Sipila19a] and ran a parameter-space exploration, testing the effects of multilayer (three-phase) ice chemistry [@Sipila16] and direct proton/deuteron hop [@Sipila19b] as opposed to full scrambling in proton-donation reactions [@Sipila15b] on the various column densities. As an example of the results, Fig. \[fig:IRAS16293\_bestfit\] shows a comparison of the beam-convolved modelled and observed column densities and column density ratios using the multi-layer ice model and full scrambling for proton-donation reactions. The chemical model reproduces very well the column density of $o$- and the / ratio, but fails to reproduce the / ratio as well as the and (*ortho* and *para*) column densities. Other parameter combinations reproduce instead very well the and (*ortho* and *para*) column densities, but strongly 