Heats of formation of alkali and alkaline earth oxides and hydroxides: Some dramatic failures of the G2 method

High-level ab initio calculations with a variety of G2-based methods have b een used to determine the structures and heats of formation of the alkali a nd alkaline earth oxides and hydroxides (M2O, MOH with M = Li, Na, and K; M 'O, M'(OH)(2) with M' = Be, Mg, and Ca). Standard G2 theory, which is norma lly very reliable for the prediction of molecular thermochemistry, is shown to be quite unsuitable for the prediction of the heats of formation of sev eral of these highly polar species, with errors greater than 100 kJ mol(-1) in some cases. Our calculations confirm that for systems containing the th ird-row atoms K and Ca, it is essential to include the 3s and 3p orbitals i n the correlation space; Interestingly, an analogous relaxed-inner-valence (denoted riv) procedure is more beneficial for the Li- and Be-containing ox ides and hydroxides than for the Na- and Mg-containing molecules. Inclusion of all orbitals in the correlation space (denoted full) generally provides only a slight further improvement, to the results. Removal of the additivi ty approximation of standard G2 theory through direct large basis set QCISD (T) calculations [e.g., G2(dir,full)] has a large effect for the oxides CaO and K2O. The QCISD(T) component of the G2 energy is poorly described for C aO, Na2O, and K2O, but this can be rectified through replacement of QCISD(T ) with CCSD(T) [e.g., G2[CC](dir,full)], For five molecules (CaO, Be(OH)(2) , Mg(OH)(2), Ca(OH)(2), and K2O) where significant discrepancies (10-30 kJ mol(-1)) remain between the best theoretical heats of formation (i.e., G2[C C](dir,full)) and experimental values, we suggest that experimental reexami nation is desirable. Structures determined at the MP2/6-311fG(3df,2p) level are in good agreement with available experimental data. Structures obtaine d at the standard MP2/631G(d) level of G2 theory are not as good, but the i mpact of using the simpler geometries on calculated heats of formation is g enerally relatively small.