The diagonal Born–Oppenheimer correction beyond the Hartree–Fock approximation

We report on evaluation of the diagonal Born–Oppenheimer correction (DBOC) to the electronic energy with Hartree–Fock (HF) and conventional correlated wave functions for general molecular systems. Convergence of both HF and configuration interaction (CI) DBOC with the one-particle basis seems to be rather fast, with triple-ζ quality correlation consistent sets of Dunning et al. sufficiently complete to approach the respective basis set limits for the DBOC of the ground state of H2 within 0.1 cm−1. Introduction of electron correlation via the CI singles and doubles method has a substantial effect on the absolute value of the DBOC for H2, H2O, and BH in their ground states (ca. +13 cm−1 out of 115 cm−1, +22 cm−1 out of 622 cm−1, and +11 cm−1 out of 370 cm−1, respectively). The effect of the correlation correction to the DBOC on relative energies is small, e.g., the barrier to linearity of water changes by ca. 1 cm−1; however, the value is difficult to converge to the ab initio limit. Based on recent results...