Third-order generalized Van Vleck perturbation theory study of the ground and lowest excited states of C2

Calculations of the potential curves of the ground (XΣg+1) and lowest two excited (BΔg1andBΣg+′1) electronic states of C2 were performed using a new third-order generalized Van Vleck multireference perturbation theory (GVVPT3), along with the GVVPT2 method and the CASSCF and MRCISD approaches, using the same 6-31G* basis set used in the Abrams and Sherrill full-CI study. Calculations in the range of 1.0-3.0 Å, which spans the equilibrium bond length and the crossing point of the (XΣg+1) and B1Δg curves, demonstrate that the deviations of the GVVPT3 curves from the full-CI curves for all states are comparable to MRCISD. GVVPT2 provides significant improvement relative to CASSCF, which already is much improved over even high-order single reference methods. Predictions of the challenging crossing point between the (XΣg+1) and B1Δg curves, in the vicinity of the BΣg+′1 avoided crossing with the ground state, is predicted to milliångstrom accuracy by MRCISD and GVVPT3, while GVVPT2 and CASSCF show deviations of about 0.07 Å.