``Benchmark Configuration Interaction Spectroscopic Constants for X 1Sigmag+ C2 and X 1Sigma+ CN+,'' M. L. Leininger, C. D. Sherrill, W. D. Allen, H. F. Schaefer, J. Chem. Phys. 108, 6717-6721 (1998).

Explicit full configuration interaction (FCI) computations in a double-zeta plus polarization (DZP) basis, involving over 100 million Slater determinants, have been performed to sample the potential energy curves of X 1Sigmag+ C2 and X 1Sigma+ CN+ for benchmark purposes. Quartic force fields have been determined at the optimized structures, and sets of anharmonic spectroscopic constants [re, omegae, Be, De, alphae, and omegaexe] have been ascertained. Analogous results obtained from high-level but inexact correlation treatments establish a CISDTQ < CCSDT < CISDTQPH ~ FCI series of increasing accuracy for the notorious X 1Sigmag C2 and X 1Sigma+ 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 mH, 0.001 Angstroms, and 4 cm-1 in the total energy, re, and omegae, 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.