Abstract The merits of cost-efficient low-level density functional theory are explored in the local spin density (LSD) approximation. Exchange and correlation are treated in the simple X α approach and the parameter α, far from being treated as a constant, is made to translate the idea that non-local exchange-correlation contributions vary in a more-or-less regular fashion when atoms join to form a molecule. Energy calculations show that the 3-21G orbital basis gives generally reliable results, with a standard deviation of around 4.5 kcal mol −1 between the calculated and experimental gas-phase enthalpies of formation examined for about 80 molecules. The results are practically the same as those obtained earlier with the 6-31G ** basis, thus illustrating the merits of the 3-21G basis in our X α calculations, but amino- and nitroaromatic molecules reveal that, in contrast to the 6-31G ** basis, 3-21G calculations overestimate thermochemical stabilities whenever the amino and nitro groups are found on adjacent carbon atoms, as in 2-nitroaniline, for example. The zero-point plus heat-content energies, ZPE+ H T − H 0, of the amino- and nitroaromatic molecules, calculated in the harmonic oscillator approximation with the 3-21G basis, obey simple additivity rules, namely a gain of 10.87 kcal mol −1 whenever a NH 2 group replaces a hydrogen atom and an increment of 2.64 kcal mol −1 for a NO 2 group replacing a hydrogen atom in benzene.