The correlation consistent composite approach (ccCA) is a model chemistry that has been shown to accurately compute gas-phase enthalpies of formation for alkali and alkaline earth metal oxides and hydroxides (Ho, D. S.; DeYonker, N. J.; Wilson, A. K.; Cundari, T. R. J. Phys. Chem. A 2006, 110, 9767).The ccCA results contrast to more widely used model chemistries where calculated enthalpies of formation for such species can be in error by up to 90 kcal mol -1 . In this study, we have applied ccCA to a more general set of 42 s-block molecules and compared the ccCA ∆H f values to values obtained using the G3 and G3B model chemistries. Included in this training set are water complexes such as Na(H 2 O) n + where n ) 1 -4, dimers and trimers of ionic compounds such as (LiCl) 2 and (LiCl) 3 , and the largest ccCA computation to date: Be-(acac) 2 , BeC 10 H 14 O 4 . Problems with the G3 model chemistries seem to be isolated to metal-oxygen bonded systems and Be-containing systems, as G3 and G3B still perform quite well with a 2.7 and 2.6 kcal mol -1 mean absolute deviation (MAD), respectively, for gas-phase enthalpies of formation. The MAD of the ccCA is only 2.2 kcal mol -1 for enthalpies of formation (∆H f ) for all compounds studied herein. While this MAD is roughly double that found for a ccCA study of >350 main group (i.e., p-block) compounds, it is commensurate with typical experimental uncertainties for s-block complexes. Some molecules where G3/ G3B and ccCA computed ∆H f values deviate significantly from experiment, such as (LiCl) 3 , NaCN, and MgF, are inviting candidates for new experimental and high-level theoretical studies.

Computing the enthalpies of formation for alkali metal and alkaline earth metal oxides (M x O) and hydroxides [M(OH) n ] using the Gaussian-n (Gn) and Weismann-n (Wn) ab initio model chemistries is difficult due to an improper treatment of core-valence electron correlation effects. Using a new model chemistry called the correlation consistent Composite Approach (ccCA), enthalpies of formation were determined for eight different alkali/alkaline earth metal oxides and hydroxides. Unlike the Gn and Wn model chemistries, which must be modified to properly account for core-valence electron correlation, the standard implementations of the ccCA provide acceptable results, and all enthalpies of formation obtained with the ccCA are within the accepted range of recommended values.