Cl 2 , Br 2 , HF, HCl, HBr, H 2 S, H 2 O, CH 4 , NH 3 , ClCH 3 , as well as the methyl radical. These species are chosen because they have precisely known heats of formation (or by definition exactly known) and are small enough such that the risk of additional errors in calculating any of these species at any LOT is low (e.g., no risk of converging to the wrong conformer).