2Σ+, 2Π and 4Π excited states of HeH: an application of the spin coupled model

Abstract

Abstract The ground and selected excited electronic states of the species H, He, HeH + and HeH are calculated using a recently developed form of a single configuration spin coupled wavefunction. It is found that the He(9s5p) and H(8s5p) GTO basis sets of McLaughlin and Thompson (J. Chem. Phys., 70 (1979) 2748) support very few excited states; augmentation of this basis with three s-type and three p-type diffuse GTOs on each atom results in a good description of between four or more excited states of a given symmetry. The states of HeH investigated here are the 2 Σ + , 2 Π valence excited states (bound relative to ionisation), and the four lowest 4 Π core excited states (lying in the doublet continuum). The high spin states require the use of basis sets for each orbital which are subsets of the total basis (so-called slide-rule basis sets) to avoid variational collapse in the iteration procedure for determining the optimum orbitals. The choice of excluded primitives in the design of suitable slide-rule basis sets is found to be more problematical in the case of molecules than it is for atoms, as some molecular high spin states dissociate into high spin atomic fragments where, sometimes, symmetry constraints are such that no slide-rule basis need be used for the separated atomic species.