Fourth‐order diagrammatic MB–RSPT calculations of the correlation energy: N2, CO, F2 and the reaction energy of the process ½F2 + ½H2 = HF

Abstract

AbstractThe correlation energies obtained by the fourth‐order diagrammatic perturbation theory were analyzed for three diatomic molecules: N2, CO, and F2. The results were compared with correlation energies obtained previously for the ten‐electron hydrides HF, H2O, and NH3. The relative importance of contributions which arise from the double excitations, from the quadruple excitations, as well as from the renormalization term was investigated. It is shown that for the diatomic molecules under study these contributions are considerably larger than for the ten‐electron hydrides not only in absolute value but also in percentage: they represent about 3, 3, and 5%, respectively, of the valence shell correlation energy obtained by the perturbation theory up to the fourth order. A careful analysis of the fourth‐order correlation effects is also presented for the reaction energy of the process ½H2 + ½F2 = HF.