The observation of an excess of ttbar production in
the threshold region, by CMS and ATLAS, has been interpreted as a
toponium contribution, i.e. from below-threshold ttbar virtual
states. The news here is the nontrivial experimental extraction of
such a signal, not its existence as such. Indeed, already 35+ years
ago an NRQCD Green's function approach was used to model the above-
and below-threshold production of ttbar pairs in pp/ppbar
collisions. The relevant cross section equations from that study are
now (re-)implemented in the Pythia 8 event generator.
While the above-threshold part is straightforward, the physical
interpretation and modelling of below-threshold events is nontrivial,
and a final prescription is cross-checked against two simpler ones.
Cross sections and some event properties are presented.