The diagonal correction to the Bornndash;Oppenheimer approximation: Its effect on the singletndash;triplet splitting of CH2and other molecular effects

摘要

The prediction of the diagonal correction to the Bornndash;Oppenheimer approximation is now possible byabinitioanalytic methods, as has recently been shown by Yarkony and Lengsfield. At the general restricted Hartreendash;Fock (GRHF) level of approximation, the procedure is straightforward: solutions of the coupled perturbed Hartreendash;Fock equations (CPHF) and some overlap integrals are all that are required. This correction is evaluated for a series of small molecules with various basis sets: H2O, H2O+, CH2, HCF, H+5, and F2. It is interesting to observe that the value of this correction (0.11 kcal) for the singletndash;triplet splitting of CH2is larger than the relativistic correction, and that the theoretical value forTnre(BO)equiv;9.23plusmn;0.20 kcal has come even closer to the bestabinitioprediction of 9.4 kcal.