Benchmark configuration interaction spectroscopic constants for X1Σg+ C2 and X1Σ+ CN+

Explicit full configuration interaction (FCI) computations in a double-zeta plus polarization (DZP) basis, involving as many as 105 million Slater determinants, have been performed to sample the potential energy curves of X (1) Sigma(g)(+) C-2 and X (1) Sigma(+) CN+ for benchmark purposes. Quartic force fields have been determined at the optimized structures, and sets of anharmonic spectroscopic constants [r(e), omega(e), B-e, (D) over bar(e), alpha(e), and omega(e) chi(e)] have been ascertained. Analogous results obtained from high-level but inexact correlation treatments establish a CISDTQ < CCSDT < CISDTQPH approximate to FCI series of increasing accuracy for the notorious X (1) Sigma(g)(+) C-2 and X (1) Sigma(+) CN+ multireference systems. The data also reveal that recent schemes for CI + PT extrapolations to the FCI limit are quite accurate, to within 0.4 mE(h), 0.001 Angstrom, and 4 cm(-1) in the total energy, r(e), and omega(e), respectively. Whether such schemes approximate FCI curves with sufficient smoothness to reproduce the anharmonic data obtained here is elevated as a challenge for future work. (C) 1998 American Institute of Physics.