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

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.