We have employed a calibration set of 192 species including the subset (109 species) of the G2/97 test set containing only the elements H through F (Table ). The errors in the total atomization energies at 0 K, ∆ AT E 0 , were determined by comparison with available experimental data or by comparison with ROCBS-QB3 energies in the absence of reliable experiments. The experimental ∆ AT E 0 used for our reference were at room temperature and were converted to 0 K using the thermal corrections from ROCBS-QB3's thermochemistry calculations.