Electron Correlation and Separated Pair Approximation in Diatomic Molecules. III. Imidogen

Abstract

The geminals of a many‐electron separated pair wavefunction are expanded in terms of their natural orbitals, and a variational procedure is developed for their determination. Within this framework, natural orbitals of the separated pair wavefunction for the ³Σ⁻ ground state of the imidogen, NH, molecule and the ${}^{4}S$ ground state of the nitrogen atom are calculated, and in order to give a pictorial appreciation of their structures, contour maps have been drawn for them. Several physical properties of the NH molecular system are calculated, namely the total energy, the dissociation energy, the equilibrium internuclear distance, the molecular potential‐energy curve, the spectroscopic constants, the molecular dipole and quadrupole moments, and various other one‐ and two‐electron expectation values. Of greatest interest is the correlation energy recovered by the separated pair wavefunction and, for this reason, a detailed analysis of the correlation is given in terms of the geminals and the natural orbitals. The study reveals that the separated pair approximation provides a suitable description of part of the electronic correlation in NH but, on the other hand, is too restrictive to yield a complete description of all the various electronic correlations in this molecular system. Nevertheless, the analysis does shed light on the nature of the omitted part of the correlation and suggests directions for future improvements.