Role of Dynamical Electron Correlation in the Differences in Bonding between CaAlH3 and MgAlH3.

The most important factor behind the intriguing differences between the geometries of the M'AlH3 (M' = Mg, Ca) molecules is shown to be dynamical electron correlation and not intramolecular Coulombic interactions, as previously thought. Spin-coupled generalized valence bond (SCGVB) calculations reveal the different bonding situations in the two molecules at their optimal geometries but do not explain why these geometries differ so much; the solution to this conundrum comes instead from detailed analysis of coupled-cluster (CCSD(T)) energies at model and optimal geometries.