Diagrammatic perturbation theory: N2 X 1Σ+g

The diagrammatic many-body perturbation theory is used to calculate the correlation energy of the nitrogen molecule in its electronic ground state. Using the algebraic approximation, the energy is evaluated through third order, including all many-body effects. [2/1] Padé approximants and variational upper bounds are constructed. For one of the perturbation expansions considered, the [2/1] Padé approximant leads to the recovery of 79.5% of the empirical correlation energy, while the variational upper bound recovers 72.0%. Three-body effects are examined in some detail. The relationships with previous work on N₂ are discussed.