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Experimental studies of the HF molecular spectrum have heretofore been unable to arrive at a suitably consistent theoretical assignment for the various measured band systems therein. To aid in this pursuit a series of ab initio C1 calculations in an AO basis containing 40 contracted gaussians has been carried out to an accuracy which is close to the full C1 limit as a result of the use of energy extrapolation techniques described elsewhere. In addition to obtaining generally quite good agreement with experimental spectroscopic constants including the dissociation energy, this treatment allows for a careful description of the change in composition of the HF ground state from ionic to covalent character as molecular stretching occurs, as well as a good representation of the upper B 1Z+ state with which it undergoes a strongly avoided crossing. The repulsive branch of the latter potential energy curve is shown to intersect the Rydberg 1Z+ manifold at relatively short bond distances, leading to a series of mixed valence-Rydberg states which are ultimately responsible for the large deviations from normal Rydberg series which are observed experimentally. Similar crossings of a---,~* and a~a* valence states with Rydberg species of 3'1/7 and/or 3Z+ symmetry are calculated to result in heavy mixing over only a relatively short range of bond distances, and thus do not produce the same magnitude of perturbations as do the 1Z+ states. Finally the calculated potential curve for the parent X2II ionic state for such Rydberg species also proves to be quite consistent with known structural data, giving independent evidence for the overall high level of accuracy of the theoretical treatment employed in the present work.

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