MC–SCF and CI calculations for the ammonia molecule

Multiconfiguration self-consistent field calculations are reported for the ammonia molecule, both with and without symmetry restrictions. Correlation energies of 0.039 and 0.080 a.u. have been obtained, respectively, for the core and the valence shell (without the symmetry restriction). It is found that the MC–SCF method is not stable with respect to the symmetry constraint, as the result of a better description of the left-right correlation. A comparison is made of configuration interaction calculations based either on canonical SCF or MC–SCF or approximate natural orbitals. Comparable values of the correlation energy are obtained from CI calculations which are based on the same number of MC–SCF orbitals or approximate natural orbitals. Analysis in terms of pair correlation energies shows that the approximate natural orbitals achieve less of the intrapair and more of the interpair correlation energy than the MC–SCF orbitals. Strongly occupied natural orbitals are close to the corresponding SCF orbitals, both being more delocalized than the MC–SCF orbitals. The weakly occupied NO and MC–SCF orbitals are comparatively much closer.