Spin-coupled VB theory of molecular electronic structure. Ground and low-lying 1Σ+ states of CH+

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Abstract
The spin‐coupled VB theory is applied to the ground and low‐lying 1Σ+ states of CH+. Using 500 spin‐coupled structures, the calculated values of the spectrosopic constants for the ground states De(eV)=4.14 (4.26), ωe(cm−1)=2845 (2858), Re(Å) =1.135 (1.131); experimental results are given in parentheses. The form of the ground state potential energy curve is very close to the MCSCF‐CI curve of Green et al. [Phys. Rev. A 5, 1614 (1972)], and similarly the excited states are very similar to those obtaind by MCSCF‐CI methods by Saxon et al. [J. Chem. Phys. 73, 1873 (1980)]. The only exception is in the 4 1Σ+ state below R=3.5a0 where there is an avoided intersection with a Rydberg state. At least for small systems of this kind, the spin‐coupled VB approach is capable of the same level of accuracy as the best MCSCF‐CI methods, but uses an order of magnitude fewer functions. Each state in the spin‐coupled VB calculation is dominated by one or two structures. This provides the wave function with much visuality, but without the usual attendant loss in accuracy.