Calculations of PAH anions: When are diffuse functions necessary?

The effect of including vs. excluding diffuse functions while calculating numerous parameters of PAH anions by various calculation methods is discussed. The omission of diffuse functions appears to have a negligible effect while calculating geometry parameters or total energy; thus, acceptable results may be obtained without them. The conclusions for charge density appear to be the same; however, limited results make an unambiguous claim unachievable. Calculating ¹H- and ¹³C-NMR shifts undoubtedly requires the use of these functions.