  Chevalier et al. (Science, 15 November 2019, p. 856-860) report mathematical
formulas and a table predicting the threshold and the output power of terahertz
molecular lasers based on various molecules. We show that these formulas are
not coherent with the simple model used to describe this kind of laser, and
that they largely overestimate the conversion efficiency. We suggest an
alternative calculation.
