Photoelectric heating efficiency as a function of the ionization parameter γ. The data for the dense PDRs NGC 2023 and the Orion Bar is taken from Hollenbach & Tielens (1999), the data for the Horsehead and L1630 is from Pabst et al. (2017), the data for diffuse PDRs is from Gry et al. (1992) and van Dishoeck & Black (1986), and the data for IC59 and IC63 is from Andrews et al. (2018). The red line shows the model of Bakes & Tielens (1994). The error bars for o Per, IC59 and IC63 have been omitted for clarity.