Multiconfiguration wavefunctions for excited states. Selection of optimal configurations: The b 1Σ+ and d 1Σ+ states of NH

A systematic method is developed to select a set of optimal configurations for multiconfiguration (MC) wavefunctions describing low-lying and excited states of molecules at all internuclear separations. The method consists in a sequence of configuration selections combined with MC orbital optimizations. It is applied to an MC study of the b 1Σ+ and d 1Σ+ states of NH, at internuclear separations from 1 to 15 bohr, using a double-zeta plus polarization basis set with additional special orbitals. Wavefunctions and energies are given. Calculated spectroscopic constants for the b 1Σ+ state are Re=1.982 bohr (1.975), De=4.496 eV (4.55), and Te=2.69 eV (2.63), and for the d 1Σ+ state Re=2.106 bohr (2.11), De=4.491 eV (4.667), and Te=11.24 eV (11.06), with experimental values in parentheses.