Extrapolation to the complete-basis-set limit and the implications of avoided crossings: The X 1Σg+, B 1Δg, and B' 1Σg+ states of C2

The uniform singlet- and triplet-pair extrapolation scheme and its generalized variant are utilized to describe bond breaking of the C-2 molecule in the ground electronic state, as well as in two other excited states of the same spin symmetry by extrapolating multireference configuration interaction energies calculated with the correlation consistent basis sets of Dunning to the complete-basis set limit. It is shown that the correlation energy changes drastically (with a symmetric sigmoidal-type shape) near the geometry (R similar to 1.6 A) where the two lowest (1)Sigma(+)(g) adiabatic states show the avoided crossing. The implications of such a feature on extrapolating the dynamical correlation are then analyzed, and the attributes of the extrapolated ground electronic state shown to be in excellent agreement with the most recent experimental data available in the literature.