The Hartree-Fock-Heitler-London method, III: Correlated diatomic hydrides

In a recently proposed model, called Hartree-Fock-Heitler-London (HF-HL) (Corongiu, G. J. Phys. Chem. A 2006, 110, 11584), the molecular wave function was variationally obtained by merging two traditional models, Hartree-Fock (HF) and Heitler-London (HL). In the new method, the non-dynamical correlation energy-which includes state avoided crossing-is explicitly calculated with a few configurations. In this work the dynamical correlation energy for diatomic hydrides of the first and second period is computed both ab initio, via short MC-HF and MC-HL expansions-including ionic and excited covalent structures-and semiempirically, using the Coulomb hole algorithm, a density functional proposed by Clementi in the early 1960s. The Coulomb Hole correction is applied to HF and HF-HL functions, and, departing from tradition, also to HL functions. Few ab initio HF-HL configurations with inclusion of ionic structures yield reasonable binding energies not only for the hydrides considered but also for the van der Waals HeH molecule. The computed binding energies (in kcal/mol) from HF-HL functions corrected with the Coulomb hole functional are as follows: 109.48 (109.48) for H21Sigma+g]; 0.01 (0.01) for HeH 2Sigma+]; 59.22 (58.00) for LiH 1Sigma+], 49.55 (49.83) for BeH 2Sigma+], 86.77 (84.1) for BH 1Sigma+], 82.65 (83.9) for CH 2Pi], 81.57 (80.5) for NH 3Sigma-], 107.18 (106.6) for OH 2Pi], and 140.91 (141.5) for HF 1Sigma+]; experimental values are given in parentheses. The computed total energies are in good agreement with exact nonrelativistic values. The combined availability of the correlation and binding energies from HF, HL, and HF-HL models allows a novel analyses on the hydrides chemical bond, in agreement with accepted physical chemistry concept derived from MO and VB theories.