Instinct.

Ab initiocomputation of the broadening of water rotational lines by molecular hydrogen

“…f ←i (T ). Following Baranger [17], Schaefer & Monchik [20,21] and Green [22], we have, for a rotational transition of H 2 O (j f , τ f ← j i , τ i ), assuming that H 2 remains in an unchanged j 2 state:…”