Optimized density functionals from the extended G2 test set

A recently suggested procedure for the systematic optimization of gradient-corrected exchange-correlation functionals A. D. Becke, J. Chem. Phys. 107, 8554 (1997) has been applied to the extended G2 test set L. A. Curtiss et al., J. Chem. Phys. 106, 1063 (1997), which consists of the standard heats of formation of 148 molecules. The limit of reproduction of the experimental data in this test set is found to be 1.78 kcal/mol mean absolute error, with a maximum of 8.89 kcal/mol error for the ozone molecule. This compares rather well with previous results for G2 theory itself (1.58 and 8.2 kcal/mol, respectively). We show that fair stability can be obtained by our optimization procedure.