Correlation-consistent configuration interaction: Accurate bond dissociation energies from simple wave functions

We have developed a general method employing relatively small but well-defined CI expansions for calculating accurate bond energies [e.g., errors of 1.4 kcal/mol (1.3%) for the C-H bond energy in CH₄ and 4.9 kcal/mol (2.7%) for the C=C bond energy in ethylene]. The approach includes in a systematic way all correlations involving orbitals that change significantly during bond breakage. The CI expansion truncates rapidly, enabling the application of this technique to polyatomic molecules for which normal correlation approaches would be prohibitively expensive. Thus the bond energy for BH is calculated to within 0.3 kcal/mol of the full CI value but incorporating less than 0.1% of the spin eigenfunctions. Smooth dissociaton to the correct adiabatic limit by the CCCI method is demonstrated for the C=C bond of ethylene. The advantage of CCCI is illustrated for C₂F ₄, where a full CI would involve ∼7 × 10²² spatial configurations, but only 1719 are used in CCCI. Here we predict a C=C bond energy for C₂F₄ of D_e (F ₂C=CF₂) = 68.3 ± 2.5 (D₂₉₈ = 64.6 ± 2.5) kcal/mol. Experimental values range from 53 to 76 kcal/mol.