Correlation Energy of Atomic Nitrogen

The Brueckner-Goldstone many-body perturbation theory is applied to study the electronic structure of the atomic nitrogen in its ground ^4S state. A complete orthonormal set of the V^N-1 single-particle states with angular symmetry up to l=5 is explicitly used to calculate the correlation-energy diagrams. The contribution from higher l (>5) is estimated by a hydrogenic approximation of the continuum wave functions. Our final correlation energy is -0. 1895 0. 003 a. u. , as compared with -0. 1886 0. 0094 a. u. from the semiempirical estimate made by Veillard and Clementi. The sum of the exchange-core-polarization and pair-correlation-energy diagrams give a total of -0. 207 28 a. u. , which is 110% of the semiempirical estimate. This contains a contribution of -0. 006 10 a. u. (3. 2%) from l>3 states and -0. 058 25 a. u. (31%) from the excitations into the valence 2p orbitals, the latter arising from the open-shell nature of the nitrogen atom. The overestimate (10%) of the correlation energy is remedied by the pair-pair correlation of 0. 011 37 a. u. (6%) and the many-electron (three and four) effect of 0. 006 42 a. u. (3. 4%). A comparison with earlier configuration-interaction results is also made.