Theoretical prediction of the potential curves for the lowest-lying states of the isovalent diatomics CN+, Si2, SiC, CP+, and SiN+ using the a b i n i t i o MRD-CI method

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Abstract
Large‐scale CI calculations are reported for the potential curves of the isovalent series of diatomic systems CN+, Si2, SiC, CP+, and SiN+ in their lowest electronic states. The standard AO basis sets employed are of double zeta plus polarization quality and the CI method used is of the multireference double‐excitation (MRD‐CI) variety including individualized configuration selection and energy extrapolation. By including up to 17 reference species to generate the MRD‐CI spaces (of orders up to 150 000) and by supplementing the AO basis with f functions it is found that the ground state of CN+ is the π4 1Σ+ species (as in isovalent C2) falling 0.1 eV below the σπ3 3Π state. This result is in significant disagreement with earlier theoretical predictions on this point, which have generally tended to place the 3Π state at least 0.3 eV below 1Σ+. The importance of using more than a single reference configuration in the CI calculations is underscored in this example. For Si2 a similar nearly isoenergetic relationship is noted for its lowest two electronic states, but in this case the competing states are 3Πu and 3Σg−, with 1Σg+ found to lie 0.7 eV higher in this spectrum. The mixed first‐ and second‐row systems SiC and CP+ both show a clear 3Π ground state, well separated from both 1Σ+ and 3Σ−, but in SiN+ the absolute energy minimum is found to occur for 3Σ−, whereby the wide variations in the relative stabilities of all these states from one system to another is seen to be a consequence of the weakening of π bonding relative to σ as second‐row atomic character is introduced into these molecules. Finally generally good agreement is observed between calculation and experiment in this study, with errors of 0.02 bohr and 70 cm−1 being indicated for known bond lengths and stretching frequencies.