Analysis of correlation consistent wavefunctions: H₃XH bond energies (X=C, Si and Ge)

We have analyzed the CCCI wavefunction for the calculation of bond dissociation energies. In the first part, CCCI is reexpressed as a non-orthogonal valence bond CI wavefunction which describes bond formation as the mixing of one covalent and two ionic parent configurations. Every other configuration is considered as an improvement of one of these three building blocks. They are grouped into different classes, the contribution of which are successively analyzed. We found the dynamical correlation of ionic structures (only one for a polar bond such as HSiH₃ or HGeH₃) to be of prime importance in obtaining accurate dissociation energies. In the second part, we improve CCCI by extending both the one- and N-electron bases, which lead to an increase and decrease of the computed bond energies, respectively. We thus conclude that the high accuracy of CCCI at the DZP level results from a cancellation of limited errors. The highest level used is uniformly accurate and yields D_e of 110.9, 96.3 and 92.5 kcal/mol for CH₄, SiH₄ and GeH₄ respectively.