Characterization of the X̃2A1 and ã4A2 electronic states of CH2+

The X^2 A1 and ã^4A2 electronic states of the methylene cation, CH2+, were investigated using the coupled cluster method with singles, doubles, and perturbatively applied triples [CCSD(T)] with Dunning's correlation consistent polarized valence basis set series (cc-pVXZ, where X=T, Q, and 5), core-valence basis sets (cc-pCVXZ, where X=T and Q), and augmented basis sets (aug-cc-pVXZ, where X=Q and 5). Explicit computation of the full set of triples (CCSDT) was also performed with the cc-pVTZ basis set. The most reliable equilibrium structures of re=1.094A and Θe=140.4° (X^2 A1) and re=1.190A and Θe=77.1° (ã^4 A2) were obtained at the CCSD(T)/aug-cc-pV5Z level. The X^2 A1-ã^4A2  classical energy separation is predicted to be 86.9 kcal/mol (30400cm^-1, 3.77 eV) at the CCSD(T)/cc-pCVQZ level of theory, and the zero-point vibrational energy corrected value is 84.5 kcal/mol (29500cm^-1, 3.66 eV).