Third‐order quasidegenerate many‐body perturbation theory calculations for valence state correlation energies of the nitrogen and oxygen atoms and their ions

Third order quasidegenerate many‐body perturbation theory (QDMBPT) is used to calculate all [2 s , 2 p ] valence state correlation energies of nitrogen and oxygen for all numbers of valence electrons, N v . The average deviations from experiment for all energies are 0.31 eV for nitrogen and 0.26 eV for oxygen. Rather than using the conventional QDMBPT treatment in which the N N electron matrix elements of the appropriate effective Hamiltonian ( H H ) are calculated directly, H H is separated into a sum of individual many‐electron operators which are independent of N v . The effective many‐electron integrals for a given species are therefore calculated once and then used to construct H H for all values of N v . This two step procedure has the special virtue of yielding all valence state correlation energies for all valence ions from a single calculation.