Binding energy of the envelope of a post-main-sequence 30 M_⊙ model with a metallicity of Z_⊙/10 computed with increasing spatial and temporal resolution. For each track we indicate the average number of zones in the model as well as the steps taken in the simulation. The lowest resolution track shown corresponds to our default setup. For comparison, we include the binding energies obtained using our default resolution together with the fits of Xu & Li (2010) and Claeys et al. (2014). Before the formation of a convective envelope, the binding energies computed at X = 0.1 and X = 0.3 almost overlap.