With that many words, it seems likely that the number of new words would eventually drop to near zero and Heaps’ law would not be applicable.
Figure 4.4 shows a plot of vocabulary growth for GOV2 together with a graph of Heaps’ law with k = 7.34 and β = 0.648.
Heaps’ law provides a good fit for this data, although the parameter values are very different than those for other TREC collections and outside the boundaries established as typical with these and other smaller collections.