The diatomic neutral oxides and their ions, MO(0,+/-), M = Sc, Ti, Cr, and Mn, have been studied through multireference configuration interaction and coupled-cluster methods. With the purpose to paint a more comprehensive and detailed picture on these not so easily tamed systems, we have constructed complete potential energy curves for a large number of states of all MO(0,+/-)'s reporting structural and spectroscopic properties. Our overall results are in very good agreement with the, in general limited, experimental data. The always difficult to be pinpointed "nature of the chemical bond" becomes more recondite for these highly open ionic-covalent species. We have tried to give some answers as to the bonding interactions using simple valence-bond-Lewis diagrams in conjunction with Mulliken populations and the symmetry of the in situ atoms. It is our belief that, particularly for this kind of molecule, molecular orbital concepts are of limited help for a consistent rationalization of the bond formation.

The diatomic molecule vanadium oxide, VO, and its charged species VO+ and VO- were studied by multireference and coupled cluster methods in conjunction with large basis sets. The investigation of 22 states and the construction of 21 full potential energy curves allowed for a detailed understanding of the electronic structure of these species. Our best binding energies for the ground states of VO (X4Sigma-), VO+ (X3Sigma-), and VO- (X3Sigma-) were De = 150, 138, and 143 kcal/mol, respectively, in harmony with the corresponding experimental values. For both species VO and VO+ and for all states studied, the bonding showed a strong ionic character conforming to the models V+O- and V2+O-.

With the high accuracy afforded by the sextuple correlation consistent basis set of Dunning, we have calculated energy levels, dissociation energies, equilibrium distances, and other spectroscopic constants for eleven valence and four Rydberg states of the CH radical. Comparisons with experimental and previous theoretical results are made for each state that has been treated. An understanding of their binding is attempted by means of simple valence bond-Lewis diagrams.

Employing multireference variational methods ͑MRCI͒, we have constructed full potential-energy curves for the ground state (X 3 ⌬) and forty excited states of the diatomic carbide, FeC. For all states we report potential-energy curves, bond lengths, dissociation energies, dipole moments, and certain spectroscopic constants, trying at the same time to get some insight on the bonding mechanisms with the help of Mulliken populations and valence-bond-Lewis diagrams. For the X 3 ⌬ state at the MRCI level of theory, we obtain a dissociation energy D e ϭ86.7 kcal/mol at a bond length r e ϭ1.581 Å. These values compare favorably to the corresponding experimental ones, D e ϭ91.2Ϯ7 ͑upper limit͒ kcal/mol and r e ϭ1.5924 Å. The first excited state ( 1 ⌬) is predicted to be 9.7 kcal/mol above the X-state as compared to an experimental value of 9.786 kcal/mol.

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