Configuration Interaction Study of the X 3Σ−, a 1Δ, and b 1Σ+ States of NH

Abstract

Using a (3s, 2p, 1d / 2s, 1p) basis set of contracted Slater-type functions and an iterative natural orbital scheme, ab initio valence configuration interaction studies have been done on the lowest three states of the imidogen radical at eight internuclear separations. Included in the CI were those configurations differing by zero, one, or two space orbitals from the Hartree–Fock configuration, except that the 1σ orbital was held doubly occupied. The size of the CI varied from 259 (1Δ) to 418 (3Σ−). For the ground state the computed total energy lies below that reported in any previous calculation except the 3379 configuration wavefunction of Bender and Davidson. From the potential curves thus obtained, the spectroscopic constants re, ωe, ωexe, Be, and αe are calculated, and compare well with the available experimental constants. The molecular splittings are calculated to be 2.00 eV (X 3Σ− − a 1Δ) and 0.79 (a 1Δ − b 1Σ+), but when the discrepancy between calculated and experimental atomic limits is taken into account these splittings are estimated as 1.47 eV and 1.02 eV, the latter being close to the experimental value of 1.05 eV. Dissociation energies (De) for the X 3Σ−, a 1Δ, and b 1Σ+, states are calculated as 3.06 eV, 3.97 eV, and 4.13 eV, respectively. For comparison, the Hartree–Fock dissociation energy for the X 3Σ− state is 2.10 eV and Gaydon's experimental value os 3.41 ± 0.16 eV. The occupation numbers and most important configurations are given at several internuclear distances for each state.